import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        String str = sc.next();
        char[] c = str.toCharArray();

        Map<Character, Integer> map = new HashMap<>();
        for (int i = 0; i < c.length; i++) {
            if (map.containsKey(c[i])) {
                map.put(c[i], map.get(c[i]) + 1);
            } else {
                map.put(c[i], 1);
            }
        }

        int max=0;
        for (char key:map.keySet()){
            max=Math.max(map.get(key),max );
        }

        for (char key:map.keySet()){
            if (map.get(key)==max){
                System.out.print(key);
            }
        }


    }
}