public class MostFrequentSubstring {
    public static void main (String[] args) throws IOException {
        CBHT<String,Integer> tabela = new CBHT<String,Integer>(300);
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        String word = br.readLine().trim();
        String sub;
        int max = 0; String maxSub = "/";
        for (int c = 0; c < word.length(); c++) {
            for (int i = 1; i <= word.length() - c; i++) {
                sub = word.substring(c, c + i);
                //System.out.println(sub);
                int idx = 0, f = 0, res = -1;
                while (true) // (true)
                {
                    idx = word.indexOf(sub, idx);
                    if (idx == -1)
                        break;
                    //System.out.println(sub + " na idx: " + idx);
                    f++;
                    idx += sub.length();
                }
                int lexi = sub.compareTo(maxSub);
                if (f > max
                        || (f == max&&(maxSub.length() < sub.length()))
                        || (lexi < 0 && (maxSub.length() < sub.length())))
                {
                    max = f;
                    maxSub = sub;
                }
                //System.out.println(sub + ": " + f);
                tabela.insert(sub, f);
            }
        }

        System.out.println(maxSub);
        //System.out.println(tabela);
    }
}