#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;
}
