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 sun.font.DelegatingShape;
import sun.misc.IOUtils;
import sun.nio.cs.KOI8_U;

import javax.smartcardio.ATR;
import java.io.*;
import java.lang.reflect.Array;
import java.util.*;
import java.math.*;
import java.lang.*;

public class AA implements Runnable {


    public void run() {
        InputReader sc = new InputReader();
        PrintWriter out = new PrintWriter(System.out);
        int i=0,j=0,k=0;
        int t=0;
        //t=sc.nextInt();
        for(int testcase = 0;testcase < t; testcase++)
        {
        }

        String str=sc.next();
        int n=str.length();
        char temp=str.charAt(0);
        for (i=1;i<n;i++)
        {
            if (temp<str.charAt(i))
            {
                temp=str.charAt(i);
            }
        }
        for (i=0;i<n;i++)
        {
            if (temp==str.charAt(i))
            {
                out.print(temp);
            }
        }




//================================================================================================================================
        out.flush();
        out.close();
    }

//================================================================================================================================
public static int[] sa(int n,InputReader sc)
{
    int inparr[]=new int[n];
    for (int i=0;i<n;i++)
        inparr[i]=sc.nextInt();
    return inparr;
}

    public static long gcd(long a,long b){
        return (a%b==0l)?b:gcd(b,a%b);
    }
    private static long lcm(long a, long b)
    {
        return a * (b / gcd(a, b));
    }
    public int egcd(int a, int b) {
        if (a == 0)
            return b;

        while (b != 0) {
            if (a > b)
                a = a - b;
            else
                b = b - a;
        }

        return a;
    }



    public int countChar(String str, char c)
    {
        int count = 0;
        for(int i=0; i < str.length(); i++)
        {    if(str.charAt(i) == c)
            count++;
        }
        return count;
    }
    static int binSearch(Integer[] inparr, int number){
        int left=0,right=inparr.length-1,mid=(left+right)/2,ind=0;
        while(left<=right){
            if(inparr[mid]<=number){
                ind=mid+1;
                left=mid+1;
            }
            else
                right=mid-1;
            mid=(left+right)/2;
        }
        return ind;
    }
    static int binSearch(int[] inparr, int number){
        int left=0,right=inparr.length-1,mid=(left+right)/2,ind=0;
        while(left<=right){
            if(inparr[mid]<=number){
                ind=mid+1;
                left=mid+1;
            }
            else
                right=mid-1;
            mid=(left+right)/2;
        }
        return ind;
    }
    static class Pair
    {
        int a,b;
        Pair(int aa,int bb)
        {
            a=aa;
            b=bb;
        }
        String get()
        {
            return a+" "+b;
        }
        String getrev()
        {
            return b+" "+a;
        }
    }

    static boolean isPrime(long n) {
        if(n < 2) return false;
        if(n == 2 || n == 3) return true;
        if(n%2 == 0 || n%3 == 0) return false;
        long sqrtN = (long)Math.sqrt(n)+1;
        for(long i = 6L; i <= sqrtN; i += 6) {
            if(n%(i-1) == 0 || n%(i+1) == 0) return false;
        }
        return true;
    }
    static long factorial(long n)
    {
        if (n == 0)
            return 1;
        return n*factorial(n-1);
    }
//================================================================================================================================

    static class InputReader
    {
        BufferedReader br;
        StringTokenizer st;

        public InputReader()
        {
            br = new BufferedReader(new
                    InputStreamReader(System.in));
        }

        String next()
        {
            while (st == null || !st.hasMoreElements())
            {
                try
                {
                    st = new StringTokenizer(br.readLine());
                }
                catch (IOException  e)
                {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        int nextInt()
        {
            return Integer.parseInt(next());
        }

        long nextLong()
        {
            return Long.parseLong(next());
        }

        double nextDouble()
        {
            return Double.parseDouble(next());
        }

        String nextLine()
        {
            String str = "";
            try
            {
                str = br.readLine();
            }
            catch (IOException e)
            {
                e.printStackTrace();
            }
            return str;
        }
    }

    public static void main(String args[]) throws Exception {
        new Thread(null, new AA(),"Main",1<<27).start();
    }
}
