#include <bits/stdc++.h>   using namespace std;   int main() {     int n, p = 0, ans = 0;     string s, dvg, answ, r_dvg;     cin >> n >> s;     for (int i = 0; i < n - 1; i++) {       dvg = s.substr(i, 2);       for (int j = i; j < n - 1; j++) {         r_dvg = s.substr(j, 2);         if (dvg == r_dvg) {           p++;         }       }       if (p > ans) {         ans = p;         answ = dvg;       }       p = 0;     }     cout << answ << endl;     return 0;   }   