#include <bits/stdc++.h>
using namespace std;
void solve() {
  int n;
  string s;
  cin >> n >> s;
  map<string, int> cnt;
  for (int i = (0); (1) > 0 ? i < (n - 1) : i > (n - 1); i += (1)) {
    ++cnt[s.substr(i, 2)];
  }
  int max_val = -1;
  string max_str = "";
  for (auto x : cnt) {
    if (x.second > max_val) {
      max_val = x.second;
      max_str = x.first;
    }
  }
  cout << max_str;
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  solve();
  return 0;
}
