import java.util.*;
import java.io.*;
import java.math.*;
import java.util.stream.Stream;

public class Main{
    /* 入出力 */
    static BufferedReader buff = new BufferedReader(new InputStreamReader(System.in));
    static StringTokenizer st;
    static PrintWriter output = new PrintWriter(System.out);
    static Scanner sc = new Scanner(System.in);
    /* 定数 */
    static final int [] y4 = {0,1,0,-1};
    static final int [] x4 = {1,0,-1,0};
    static final int  INF1 = Integer.MAX_VALUE;
    static final long INF2 = Long.MAX_VALUE;
    static final long MOD1 = (long)1e9+7;
    static final long MOD2 = 998244353;
    /* Main */
    public static void main(String[] args) throws IOException{
        int [] cnt = new int[26];
        char [] m = new char[26];
        for(char i = 'a'; i <= 'z'; i ++) m[i-'a'] = i;
        String s = sc.next();
        int n = s.length();
        int what = 0;
        char k = 'a';
        for(int i=0;i<n;i++) cnt[s.charAt(i) - 'a']++;
        for(int i=0;i<26;i++) {
            if(cnt[i] > 0) {
                what = i;
                break;
            }
        }
        for(int i=0;i<=cnt[what];i++) output.print(m[what]);
        output.flush();
    }
}