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".
from __future__ import division
import sys
from collections import namedtuple #x=namedtuple('point','x y')
from math import *
from collections import deque, OrderedDict #append #popleft
from fractions import gcd
from copy import copy ,deepcopy
from collections import Counter #Counter(list)
import re #re.split("[^a-zA-Z]",text)
from decimal import Decimal
# from functools import lru_cache #@lru_cache(maxsize = None)
"""
#reduce(func,list)
#map(func,list)
#filter(func,list)
#xor=lambda x,y :x^y
#sign = lambda x: math.copysign(1, x)
#list.reverse() 
#list.sort() list=sorted(list)
list.sort(key=operator.itemgetter(1,2))
#reverse word word[::-1]
#word.islower()
#word.lower() word.upper()
x = x1 if exp1 else x2
any([false,true])
all([true,false])
"a".isalpha()
"1".isdigit()
int(str,base)
"""


fin=sys.stdin ;fout=sys.stdout 
# fin=open('../in','r') ; fout=open('../out','w')
def readline():
    return fin.readline().strip()
def readstrings():
    return fin.readline().strip().split(' ')
def writeline(value):
    fout.write(str(value))
    fout.write("\n")
def read_integers():
    return [int(x) for x in fin.readline().strip().split(' ')]
def read_integer():
    return int(fin.readline().strip())
def end():
    sys.exit()
def readbig():
    return [Decimal(x) for x in fin.readline().strip().split(' ')]

    
w=readline()
w=list(w)
ch=max(w)
writeline(ch*w.count(ch))
            
    