#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  string s;
  cin >> s;
  map<string, int> gram;
  for (int i = 0; i < n - 1; i++) ++gram[s.substr(i, 2)];
  int cnt = 0;
  string ans = "";
  for (auto e : gram)
    if (e.second > cnt) cnt = e.second, ans = e.first;
  cout << ans;
}
