#include <bits/stdc++.h>
using namespace std;
map<string, int> mp;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  string s;
  string ans;
  int cnt = 0, n;
  cin >> n;
  cin >> s;
  int ln = s.size();
  for (int i = 0; i < ln - 1; i++) {
    char t[2];
    t[0] = s[i];
    t[1] = s[i + 1];
    string tmp(t);
    mp[tmp]++;
    if (cnt < mp[tmp]) {
      cnt = mp[tmp];
      ans = tmp;
    }
  }
  cout << ans[0] << ans[1] << endl;
  return 0;
}
