#include <bits/stdc++.h>
using namespace std;
int main(int argc, char const *argv[]) {
  ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  int n;
  cin >> n;
  string s;
  cin >> s;
  unordered_map<string, int> umap;
  int _max = INT_MIN;
  string ms;
  for (int i = 0; i <= n - 2; i++) {
    if (++umap[s.substr(i, 2)] > _max) {
      _max = umap[s.substr(i, 2)];
      ms = s.substr(i, 2);
    }
  }
  cout << ms << endl;
  return 0;
}
