This problem's actual name, "Lexicographically Largest Palindromic Subsequence" is too long to fit into the page headline.

You are given string s consisting of lowercase English letters only. Find its lexicographically largest palindromic subsequence.

We'll call a non-empty string s[p1p2... pk] = sp1sp2... spk (1 ≤  p1 < p2 < ... < pk ≤  |s|) a subsequence of string s = s1s2... s|s|, where |s| is the length of string s. For example, strings "abcb", "b" and "abacaba" are subsequences of string "abacaba".

String x = x1x2... x|x| is lexicographically larger than string y = y1y2... y|y| if either |x| > |y| and x1 = y1, x2 = y2, ..., x|y| = y|y|, or there exists such number r (r < |x|, r < |y|) that x1 = y1, x2 = y2, ..., xr = yr and xr + 1 > yr + 1. Characters in the strings are compared according to their ASCII codes. For example, string "ranger" is lexicographically larger than string "racecar" and string "poster" is lexicographically larger than string "post".

String s = s1s2... s|s| is a palindrome if it matches string rev(s) = s|s|s|s| - 1... s1. In other words, a string is a palindrome if it reads the same way from left to right and from right to left. For example, palindromic strings are "racecar", "refer" and "z".

Input

The only input line contains a non-empty string s consisting of lowercase English letters only. Its length does not exceed 10.

Output

Print the lexicographically largest palindromic subsequence of string s.

Examples

Input

radar


Output

rr


Input

bowwowwow


Output

wwwww


Input

codeforces


Output

s


Input

mississipp


Output

ssss

Note

Among all distinct subsequences of string "radar" the following ones are palindromes: "a", "d", "r", "aa", "rr", "ada", "rar", "rdr", "raar" and "radar". The lexicographically largest of them is "rr".
#q1
'''
for _ in range(int(input())):
    l=list(map(int,input().split()))

    x=l[1]
    y=l[2]
    z=l[2]

    print(x,y,z)
'''  
#q2
'''
for _ in range(int(input())):

    x,n,m=map(int,input().split())
    
    while(x>20 and n>0 and x > x//2+10):
        x=x//2+10
        n-=1

    if(x <= m*10):
        print("YES ")
    else:
        print("NO ")
'''

#q3 
'''
def swap(s,j,mi):
    l=list(s)
    temp=l[j]
    l[j]=l[mi]
    l[mi]=temp

    s=''.join(l)

    return s

for _ in range(int(input())):

    s,t=input().split()

    k=0
    for i in range(len(s)):
        if ord(s[i]) < ord(t[i]):
            k=1
            break

        elif ord(s[i]) == ord(t[i]):
            continue

        else:
            mi=i
            for j in range(i+1,len(s)):
                if ord(s[j]) < ord(t[mi]):
                    s=swap(s,j,mi)
                    k=1
                    break
            break
    if k==1:
        print(s)

    else:
        print('---')

'''

#q4
'''
import math

def lcm(n):
    p=n
    t=int(math.sqrt(n))+1
    l=[0]*n
    l[0]=1
    
    while n%2 == 0:
        if n//2==2:
            l[2-1]+=1
        else:
            l[2-1]+=1
            l[(n//2)-1]+=1
        n=n//2

    for i in range(3,t):
        while n%i==0:

            if n//i==i:
                l[i-1]+=1
            else:
                l[i-1]+=1
                l[(n//i)-1]+=1
                
            n=n//i

    print(l)
    d=set()
    for i in range(p):
#        print('1')
        if l[i]!=0:
            d.add(pow((i+1),l[i]))
#            print('2')

    print(d)
    return list(d)
            
n=int(input())

f=lcm(n)
print(f)

if len(f)==1 or len(f)==2:
    print('1',n)

else:
    t1=f[1]
    t2=f[2]
    k=min(t1,t2)
    for i in range(3,len(f)):
        if t1==k:
            t1*=k
        else:
            t2*=k
        k=min(t1,t2)

    print(t1,t2)
        
'''     
        
#q5
'''
for _ in range(int(input())):

    string=input()
    i=0
    de=0
    z=[]
    
    while i<len(string)-2:

        if string[i:i+5]=="twone":
            de+=1
            z.append(i+2)
            i+=5

        elif string[i:i+3]=="one":
            de+=1
            z.append(i+1)
            i+=3

        elif string[i:i+3]=="two":
            de+=1
            z.append(i+1)
            i+=3       
            
        else:
            i+=1

    print(de)
    for i in range(len(z)):
        print(z[i]+1,end=" ")

    print()
'''        

#string modification ****
'''
for _ in range(int(input())):
    n=int(input())

    string=input()

    l=list(string)
    
    for k in range(n):
        first=n-k

        for i in range(first):
            second=i+k

            l[i:second+1:1]=l[second:i-1:-1]
            print(l)

    print(l)

'''

#remove adjacent
'''
n=int(input())

string=input()
l=list(string)
#print(l)
de=0

while(1):
    q=l
    a=[]
    t=0
    
    for i in range(1,len(l)):
        a.append(ord(q[i])-ord(q[i-1]))
    
    if((1 in a) or (-1 in a)) :
        for i in range(len(a)):
            if a[i]==-1:
                l.pop(i-t)
                t+=1
                de+=1
#                print(l)

            elif a[i]==1:
                l.pop(i+1-t)
                t+=1
                de+=1
#                print(l)

        print(l)
        
    else:
        break
print(de)
'''            


#Yet Another Counting Problem
'''
def gcd(a,b):
    if b==0:
        return a
    else:
        return gcd(b,a%b)

    
def solve(r,lcm,b):

    no_of_term=r//lcm
    product=no_of_term*lcm

    tot=r

    if product!=0:

        tot-=min(b,(r-product+1))
        tot-=(no_of_term-1)*b
        
    return tot-min(r,b-1)

for _ in range(int(input())):

    a,b,q=map(int,input().split())
    if a>b:
        a,b=b,a
        
    gc=gcd(a,b)
    product=a*b
    lcm=product//gc
    
    for i in range(q):
        l,r=map(int,input().split())

        ans1=solve(r,lcm,b)
        if l>=b:
            ans2=solve(l-1,lcm,b)
            ans=ans1-ans2
        else:
            ans=ans1
            
        print(ans,end=" ")
    print()
'''

#number game
'''
import math

import math

f='FastestFinger'
a='Ashishgup'
def prime(n):
    
    r=int(math.sqrt(n))+1
    
    if n%2==0 and n!=2:
        return False

    for i in range(3,r):
        if n % i ==0:
            return False
        
    return True
  
def solve(n):

    if n==0 :
        return
    elif n==1:
        return f
    
    dp=[]
    dp.append(0)
    
    
for _ in range(int(input())):

    n=int(input())
    if prime(n) and n>1:
        print("Ashishgup")
    else:
        print(solve(n))

'''

#Ciel and Flowers
'''
l=list(map(int,input().split()))
t=list(l)

su1=0

for i in range(3):
    su1+=int(l[i]/3)
    l[i]=l[i]%3
    
if min(l)!=0: 
    su1+=min(l)

q=min(t)
su2=q

for i in range(3):
    t[i]-=q

for i in range(3):
    su2+=int(t[i]/3)
    
print(max(su2,su1))
'''


#Count Subrectangles
'''
def row(n,m,k,c):

    l=[]
    for i in range(n):
        su=0
        for j in range(k):
            su+=c[i][j]
        l.append(su)

    for i in range():    

n,m,k=map(int,input().split())

a=list(map(int,input().split()))
b=list(map(int,input().split()))

c=[[0 for i in range(m)]for j in range(n)]

for i in range(n):
    for  j in range(m):
        c[i][j]=a[i]*b[j]

row(n,m,k,c)

'''  

#Ciel and Flowers
'''
a,b,c=map(int,input().split())

ans1=a//3+b//3+c//3
ans2=ans3=0

if a>0 and b>0 and c>0:
    a-=1
    b-=1
    c-=1

    ans2=a//3+b//3+c//3

    ans2+=1

if a>0 and b>0 and c>0 :
    a-=1
    b-=1
    c-=1

    ans3=a//3+b//3+c//3

    ans3+=2

print(max(ans1,ans2,ans3))
'''   

#Way Too Long Words
'''
for _ in range(int(input())):

    s=input()
    ans=""
    
    if len(s)>10:
        t=str(len(s)-2)
        ans+=s[0]+t+s[len(s)-1]

    else:
        ans+=s

    print(ans)
'''

#Progress Bar
'''
n,k,t=map(int,input().split())

a1=n*k

a2=t*a1/100

a3=int(a2/k)

s=int(a2-a3*k)

if s>0:
    for i in range(n):
        if i<a3:
            print(k,end=" ")
        elif i==a3:
            print(s,end=" ")
        else:
            print('0',end=" ")

else:
    for i in range(n):
        if i<a3:
            print(k,end=" ")

        else:
            print('0',end=" ")
    
'''

# Required Remainder
'''
for _ in range(int(input())):

    x,y,n=map(int,input().split())
    t1=n//x
    t2=t1*x
#    print(t1,t2)
    
    if y==0:
        print(t2)
    else:
        a=x-y
        
        if y+t2<=n:
            print(y+t2)
        else:
            print(t2-a)
'''   
    
'''
t1=12345//7
print(t1)
t2=t1*7
print(t2)
'''

# Multiply by 2, divide by 6
'''
import math

def factoriation(n):
    k=int(math.sqrt(n))+1
    
    l=[0]*k

    t=0
    l[0]=0
    l[1]=0
    
    
    while n%2==0:
        t+=1
        n=n//2
        
    l[2]=t

    for i in range(3,k):
        t=0
        while n%i==0:
            t+=1
            n=n//i
        l[i]=t
        
        if n==1:
            return l
        else:
            return [0,0,0,0]
        
    return l

for _ in range(int(input())):

    n=int(input())

    if n==6:
        print('1')

    elif n==3:
        print('2')

    elif n==1:
        print('0')
    
    elif n>=9:
        l=factoriation(n)

        if l[3] >= l[2] and l[3]!=0:
            ans=-1
            ans=l[2]
            l[3]=l[3]-l[2]
            l[2]=0

            ans+=l[3]*2

            print(ans)
            
        else:
            print('-1')

    else:
        print('-1')

'''

# Move Brackets
'''
for _ in range(int(input())):

    n=int(input())

    string=input()

    ans=0
    x2=0
    
    for i in range(n):
        if string[i]==')':
            if x2==0:
                ans+=1
            else:
                x2-=1

        elif string[i]=="(":
            x2+=1

    print(ans)
'''      

#Petya and Strings
'''
string1=input().lower()
string2=input().lower()

for i in range(len(string1)):

    if string1[i] > string2[i]:
        print('1')
        break
    elif string1[i] < string2[i] :
        print('-1')
        break
     
    else:
        if i==len(string1)-1:
            print('0')
            break
        else:
            continue
'''

#Helpful maths
'''
string=input()
li=list(string.split("+"))
l=[0]*3

li.sort()

for i in range(len(li)):
    if i!=len(li)-1:
        print(li[i]+'+',end="")
    else:
        print(li[i])
'''

#word capitalisation
'''
string=input()

if ord(string[0])>96:
    print(chr(ord(string[0])-32)+string[1:])

else:
    print(string)
'''

#Boy or Girl
'''
string=input()

l=[0]*26

for i in range(len(string)):
    l[ord(string[i])-97]+=1

k=0
for i in range(26):
    if l[i]>0:
        k+=1

if k%2==0:
    print('CHAT WITH HER!')
else:
    print('IGNORE HIM!')
'''

#Translation
'''
s=input()
t=input()

k=""

for i in range(len(s)-1,-1,-1):
    k+=s[i]

if k==t:
    print("YES")
else:
    print("NO")

'''

#Word
'''
string=input()
lower=0
upper=0


for i in range(len(string)):
    if ord(string[i])>96:
        lower+=1

    else:
        upper+=1

if upper>lower:
    print(string.upper())

else:
    print(string.lower())
            
'''

# Anton and Danik
'''
n=int(input())

string=input()

A=0
D=0

for i in range(n):
    if string[i]=="A":
        A+=1
    else:
        D+=1

if A>D:
    print('Anton')
elif A<D:
    print("Danik")

else:
    print("Friendship")
'''

#Football
'''
string=input()
a=b=0

for i in range(len(string)):

    if string[i]=='0':
        a+=1
        b=0

    else:
        b+=1
        a=0

    if a==7 or b==7:
        t=1
        break

    else:
        t=0

if t==1:
    print("YES")
else:
    print("NO")
'''

#Dubstep
'''
string=input()
s=""
i=0
while i<len(string)-2:

    if string[i]=="W" and string[i+1]=="U" and string[i+2]=="B":
        i+=3
        s+=" "

    else:
        s+=string[i]
        i+=1

if i>=len(string):    
    print(' '.join(s.split()))
    
else:
    s+=string[i]
    i+=1
    
    if i>=len(string):
        print(' '.join(s.split()))
    else:
        s+=string[i]
        print(' '.join(s.split()))
'''

#Two_gram
'''
n=int(input())
string=input()

res=0

for i in range(len(string)-1):
    cur=0
    for j in range(i,len(string)-1):

        if string[i]==string[j] and string[i+1]==string[j+1]:
            cur+=1

    if res < cur:

        res=cur
        ans=string[i]+string[i+1]

print(ans)
'''

#Even But Not Even
'''
for _ in range(int(input())):

    n=int(input())
    string=input()
    l=list(string)
    su=0
    
    for i in range(len(string)-1,-1,-1):
        su+=int(l[i])
        if l[i]=="0":
            l.pop(i)
#    print(su)
    if su%2!=0:
        if int(string)%2==0:
            for i in range(len(l)-1,-1,-1):
                if int(l[i])%2==0:
                    l.pop(i)
                else:
                    break
            for i in range(len(l)-2,-1,-1):
                if int(l[i])%2!=0:
                    l.pop(i)
                    break
                
            if len(l)>1:   
                print(''.join(i for i in l))

            else:
                print('-1')
                
        else:
#            print(l)
            for i in range(len(l)-2,-1,-1):
                if int(l[i])%2!=0:
                    l.pop(i)
                    break

            if len(l)>2:
                print(''.join(i for i in l))
            elif len(l)==2 and int(l[0])%2!=0:
                print(''.join(i for i in l))
            else:
                print('-1')

    else:
#        print(su)
        if int(string)%2!=0:
            print(''.join(i for i in l))
        else:
            for i in range(len(l)-1,-1,-1):
                if int(l[i])%2==0:
                    l.pop(i)
                else:
                    break
            if len(l)>1:
                print(''.join(i for i in l))
            else:
                print('-1')

'''

# Delete from the Left
'''
s=input()
t=input()
q=0


i1=len(t)-1
i2=len(s)-1
    
while i1>=0 and i2>=0:
    if s[i2]==t[i1]:
        q+=1
        i1-=1
        i2-=1
            
    else:
        break


su1=len(s)-q
#print(su1,q)
su2=len(t)-q
print(su1+su2)

'''

#Important exam
'''
n,m=map(int,input().split())
k=[]
    
for i in range(n):
    string=input()
    l=list(string)
    k.append(l)

q=list(map(int,input().split()))
su=0

for i in range(m):
    t1=t2=t3=t4=t5=0

    for j in range(n):
        if k[j][i]=="A":
            t1+=1

        elif k[j][i]=="B":
            t2+=1

        elif k[j][i]=="C":
            t3+=1

        elif k[j][i]=="D":
            t4+=1

        else:
            t5+=1

    a=max(t1,t2,t3,t4,t5)
    su+=a*q[i]

print(su)
            
'''

# Letters Rearranging
'''
for _ in range(int(input())):

    string=input()
    t=1
    j=0
    
    for i in range(len(string)-1):
        if string[i] == string[i+1]:
            t+=1
        else:
            j=i+1
            break

    if t==len(string):
        print('-1')
        
    else:
        k=0
        for i in range(len(string)//2):
            if string[i]==string[len(string)-1-i]:
                k+=2

        if len(string)%2==0 and k==len(string):
            l=list(string)
            l[0],l[j]=l[j],l[0]
            print(''.join(i for i in l))

        elif len(string)%2==1 and k==len(string)-1:
            l=list(string)
            l[0],l[j]=l[j],l[0]
            print(''.join(i for i in l))

        else:
            print(string)
'''

#Romaji
'''
string=input()
t=0

if string[len(string)-1]=='n' or string[len(string)-1]=='a' or string[len(string)-1]=='e' or string[len(string)-1]=='i' or string[len(string)-1]=='o' or string[len(string)-1]=='u':
    t=1
    for i in range(0,len(string)-1):

        if (string[i]!='n' or string[i]!='a' or string[i]!='e' or string[i]!='i' or string[i]!='o' or string[i]!='u') and (string[i+1]=='a' or string[i+1]=='e' or string[i+1]=='i' or string[i+1]=='o' or string[i+1]=='u'):
            t=1
            
        elif (string[i]=='a' or string[i]=='e' or string[i]=='i' or string[i]=='o' or string[i]=='u') and string[i+1]=='a' or string[i+1]=='e' or string[i+1]=='i' or string[i+1]=='o' or string[i+1]=='u':
            t=1
            
        elif string[i]=='n':
            t=1

        elif string[i]=='a' or string[i]=='e' or string[i]=='i' or string[i]=='o' or string[i]=='u':
            t=1
            
        else:
            t=0
            break

else:
    t=0
        
if t==0:
    print('NO')
else:
    print("YES")

'''

#String Task
'''
string=input().lower()

l=['a','e','i','o','u','y']
k=list(string)
i=0

while i <len(k):
    if k[i] in l:
        k.pop(i)
    else:
        i+=1
        
for i in range(len(k)):
    
    print('.'+k[i],end="")
'''

#chat room
'''
string=input().lower()

if len(string)<5:
    print("NO")
else:
    l=['h','e','l','l','o']
    k=list(string)
    i=0
    
    while i<len(k):
        if k[i]==l[0]:
            l.pop(0)
            i+=1

        else:
            i+=1

        if len(l)==0:
            break

    if len(l)==0:
        print("YES")
    else:
        print("NO")

'''

#cAPS lOCK
'''
string=input()

a1=a=b=0
l=list(string)

if ord(l[0])<91:
    a1+=1
else:
    a1=0
    
for i in range(1,len(string)):

    if ord(l[i])<91:
        b+=1

if a1==0 and b==len(l)-1:
    for i in range(len(l)):
        if i==0:
            print(chr(ord(l[i])-32),end="")
        else:
            print(chr(ord(l[i])+32),end="")
        
elif a1==1 and b==len(l)-1:
    for i in range(len(l)):
        print(chr(ord(l[i])+32),end="")

else:
    print(string)
    
'''

#Lecture
'''
n,m=map(int,input().split())
q1=[]
q2=[]

for i in range(m):
    a,b=input().split()
    q1.append(a)
    q2.append(b)

k=input()

l=list(k.split())

for i in range(n):

    a=q1.index(l[i])
    if len(q1[a]) > len(q2[a]):
        print(q2[a],end=" ")
    else:
        print(q1[a],end=" ")
'''

# Football
'''
l=[1,0]

t=int(input())
string=input()

for i in range(1,t):

    n=input()
    if n==string:
        l[0]+=1
    else:
        j=n
        l[1]+=1

if l[0]>l[1]:
    print(string)

else:
    print(j)
'''

#Pangram
'''
n=int(input())

string=input().lower()
l=[0]*26

if n>=26:
    
    for i in range(n):
        l[ord(string[i])-97]+=1

    if min(l)!=0:
        print("YES")
    else:
        print("NO")

else:
    print("NO")
'''

#Amusing Joke
'''
s1=input().lower()
s2=input().lower()

string=input().lower()
l=[0]*26

for i in range(len(s1)):
    l[ord(s1[i])-97]+=1

for i in range(len(s2)):
    l[ord(s2[i])-97]+=1

t=0


for i in range(len(string)):

    if l[ord(string[i])-97]!=0:
#        print(string[i],l[ord(string[i])-97])
        l[ord(string[i])-97]-=1

    else:
        t=1
        break

if t==1:
    
    print("NO")
    
elif t==0 and max(l)==0:
    print("YES")

else:
    print("NO")

'''

#Night at the Museum
'''
string=input()

su=min(ord(string[0])-97 - 0 , 26 - ord(string[0])+97 -0)

for i in range(1,len(string)):
    if string[i-1]>string[i]:
        su+=min(abs( (ord(string[i])-97) - (ord(string[i-1])-97 ) ), abs(26+ (ord(string[i])-97 ) - (ord(string[i-1])-97) ))
    else:
        su+=min(abs( (ord(string[i])-97) - (ord(string[i-1])-97 ) ), abs(26- (ord(string[i])-97 ) +(ord(string[i-1])-97) ))

print(su)
'''

#Cards
'''
n=int(input())

string=input()
l=[0]*26
q=[]


for i in range(n):
    l[ord(string[i])-97]+=1

while max(l)!=0:
    if l[13]>=2 and l[8]>=1 and l[4]>=1:
        q.append(9)
        l[13]-=2
        l[8]-=1
        l[4]-=1

    elif l[4]>=1 and l[8]>=1 and l[6]>=1 and l[7]>=1 and l[19]>=1:
        q.append(8)
        l[4]-=1
        l[8]-=1
        l[6]-=1
        l[7]-=1
        l[19]-=1

    elif l[18]>=1 and l[4]>=2 and l[21]>=1 and l[13]>=1:
        q.append(7)
        l[18]-=1
        l[4]-=2
        l[21]-=1
        l[13]-=1

    elif l[18]>=1 and l[8]>=1 and l[23]>=1 :
        q.append(6)
        l[18]-=1
        l[8]-=1
        l[23]-=1

    elif l[5]>=1 and l[8]>=1 and l[21]>=1 and l[4]>=1:
        q.append(5)
        l[5]-=1
        l[8]-=1
        l[21]-=1
        l[4]-=1

    elif l[5]>=1 and l[14]>=1 and l[20]>=1 and l[17]>=1:
        q.append(4)
        l[5]-=1
        l[14]-=1
        l[20]-=1
        l[17]-=1
        
    elif l[19]>=1 and l[7]>=1 and l[17]>=1 and l[4]>=2:
        q.append(3)
        l[19]-=1
        l[7]-=1
        l[17]-=1
        l[4]-=2

    elif l[19]>=1 and l[22]>=1 and l[14]>=1:
        q.append(2)
        l[19]-=1
        l[22]-=1
        l[14]-=1

    elif l[14]>=1 and l[13]>=1 and l[14]>=1:
        q.append(1)
        l[14]-=1
        l[13]-=1
        l[4]-=1

    else:
        q.append(0)
        l[25]-=1
        l[4]-=1
        l[17]-=1
        l[14]-=1
        

q=sorted(q,reverse=True)
for i in range(len(q)):
    print(q[i],end=" ")

'''  

#Telephone Number
'''
for _ in range(int(input())):

    n=int(input())

    string=input()
    a=n-11
    t=0
    
    if a<0:
        t=1
        
    elif a==0 and string[0]!="8":
        t=1
        
    else:
        l=list(string)

        while l[0]!='8':
            if a==0 and l[0]!=8:
                t=1
                break
                
            l.pop(0)
            a-=1

    if t==1:
        print("NO")
    else:
        print("YES")
'''

#LLPS

def subs(s):

    sub=[[]]

    for i in s:
        sub+=[lst + [i] for lst in sub]

    return sub

    
string=input()

f=subs(list(string))

q=[]

for i in range(len(f)):
    t=0
    for j in range(len(f[i])//2):
        
        if f[i][j]!=f[i][len(f[i])-j-1]:
            t=1
            break
    if t!=1:
        q.append(f[i])

max=q[0]
for i in range(1,len(q)):
    if max < q[i]:
        max=q[i]

print(''.join(i for i in max))











            
                
            
                
































































        
            
        

        









