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".
import java.util.Scanner;
import java.io.*;
import java.util.*;
import java.math.*;
import java.lang.*;
import static java.lang.Math.*;

public class main implements Runnable{

    static LinkedList <Integer> adj[];
    static int co=0,f=0;

    static void Check2(int n){
        adj=new LinkedList[n+1];
        for(int i=0;i<=n;i++){
            adj[i]=new LinkedList();
        }

    }
    static void add(int i,int j){
        adj[i].add(j);
    }
    public static void main(String[] args) throws Exception {
        new Thread(null, new main(), "Check2", 1<<26).start();// to increse stack size in java
    }
    static long mod=(long)(1e9+7);
    static  int r,g,y,b;
    static long le[];
    public void run(){
        /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
        //Scanner in=new Scanner(System.in);
        InputReader in=new InputReader(System.in);
        PrintWriter w=new PrintWriter(System.out);


        char c[]=in.next().toCharArray();

        char d='a';
        for(int i=0;i<c.length;i++)
        {
            if(c[i]>d)d=c[i];
        }
        int co=0;
        for(int i=0;i<c.length;i++)
        {
            if(c[i]==d)co++;
        }


        for(int i=0;i<co;i++){
            w.print(d);
        }











                w.close();
    }
    static class pair implements  Comparable <pair> {

        int a;
        long b;
        pair(int c,long d){
            a=c;
            b=d;
        }
        public int compareTo(pair o){
            if(this.b!=o.b)
            return -(Long.compare(this.b,o.b));
            else
                return  -(this.a-o.a);
        }



    }

   /* static void dfs(int i,int lvl,int v[]){



        v[i]=1;
        //   System.out.println(i);
        le[1]=(long)(le[1])+1l;
        if(lvl+1<=n){
            le[lvl+1]-=1l;
        }
        Iterator <Integer> ne=adj[i].iterator();
        while(ne.hasNext()){
            int k=ne.next();

            if(v[k]==0){
                v[k]=1;
                dfs(k,lvl+1,v);
            }


        }


    }*/
    static class node{

        int y;
        int val;

        node(int a,int b){

            y=a;
            val=b;

        }


    }
    static  void rec(String s,int a,int b,int n){

        if(b==n){
            System.out.println(s);
            return ;
        }
        String p=s;
        if(a>b){
            s=p+")" ;
            rec(s,a,b+1,n);
        }
        if(a<n){
            s=p+"(";
            rec(s,a+1,b,n);
        }




    }

    static class InputReader
    {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private SpaceCharFilter filter;

        public InputReader(InputStream stream)
        {
            this.stream = stream;
        }

        public int read()
        {
            if (numChars==-1)
                throw new InputMismatchException();

            if (curChar >= numChars)
            {
                curChar = 0;
                try
                {
                    numChars = stream.read(buf);
                }
                catch (IOException e)
                {
                    throw new InputMismatchException();
                }

                if(numChars <= 0)
                    return -1;
            }
            return buf[curChar++];
        }

        public String nextLine()
        {
            BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
            String str = "";
            try
            {
                str = br.readLine();
            }
            catch (IOException e)
            {
                e.printStackTrace();
            }
            return str;
        }
        public int nextInt()
        {
            int c = read();

            while(isSpaceChar(c))
                c = read();

            int sgn = 1;

            if (c == '-')
            {
                sgn = -1;
                c = read();
            }

            int res = 0;
            do
            {
                if(c<'0'||c>'9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            }
            while (!isSpaceChar(c));

            return res * sgn;
        }

        public long nextLong()
        {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-')
            {
                sgn = -1;
                c = read();
            }
            long res = 0;

            do
            {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            }
            while (!isSpaceChar(c));
            return res * sgn;
        }

        public double nextDouble()
        {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-')
            {
                sgn = -1;
                c = read();
            }
            double res = 0;
            while (!isSpaceChar(c) && c != '.')
            {
                if (c == 'e' || c == 'E')
                    return res * Math.pow(10, nextInt());
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            }
            if (c == '.')
            {
                c = read();
                double m = 1;
                while (!isSpaceChar(c))
                {
                    if (c == 'e' || c == 'E')
                        return res * Math.pow(10, nextInt());
                    if (c < '0' || c > '9')
                        throw new InputMismatchException();
                    m /= 10;
                    res += (c - '0') * m;
                    c = read();
                }
            }
            return res * sgn;
        }

        public String readString()
        {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            StringBuilder res = new StringBuilder();
            do
            {
                res.appendCodePoint(c);
                c = read();
            }
            while (!isSpaceChar(c));

            return res.toString();
        }

        public boolean isSpaceChar(int c)
        {
            if (filter != null)
                return filter.isSpaceChar(c);
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public String next()
        {
            return readString();
        }

        public interface SpaceCharFilter
        {
            public boolean isSpaceChar(int ch);
        }
    }







}