#include <bits/stdc++.h>   using namespace std;   int n;   string s, ans;   int main() {     ios_base::sync_with_stdio(0);     cin.tie(0);     cin >> n;     cin.ignore(numeric_limits<streamsize>::max(),   n );     cin >> s;     bool flag = true;     int cnt = 0, max_cnt = 0;     for (int i = 0; i < n - 1; ++i) {       string sub = s.substr(i, 2);       cnt = 0;       for (int j = 0; j < n - 1; ++j) {         flag = true;         for (int k = 0; k < 2; ++k) {           if (sub[k] != s[j + k]) flag = false;         }         if (flag) ++cnt;         if (cnt > max_cnt) {           max_cnt = cnt;           ans = sub;         }       }     }     cout << ans << endl;     return 0;   }   