import java.util.HashMap;
import java.util.*;

public class Frequency {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String s = scanner.nextLine().trim();
        Map<String, Integer> freq = new HashMap<>();
        for(int i = 0; i < s.length(); i++) {
            String tmp = "";

            for(int j = i; j < s.length(); j++) {
                tmp += s.charAt(j);

                freq.put(tmp, freq.getOrDefault(tmp, 0) + 1);
            }
        }

        String res = "";
        int bestFreq = -1;

        for(Map.Entry<String, Integer> e: freq.entrySet()) {
            String c = e.getKey();
            int val = e.getValue();

            if(val > bestFreq) {
                bestFreq = val;
                res = c;
            }
            else if(val == bestFreq) {
                if(c.length() > res.length()) {
                    res = c;
                }
                else if(c.length() == res.length()) {
                    if(c.compareTo(res) < 0) {
                        res = c;
                    }
                }
            }
        }

        System.out.println(res);

        scanner.close();
    }
}