#include <bits/stdc++.h>
const int N = (5e5);
const long long INF = (long long)(2e9) + 7;
using namespace std;
int n, k, a[N], mx;
string s, ans;
map<string, int> M;
int main() {
  ios_base ::sync_with_stdio(false);
  cin.tie(0);
  cin >> n >> s;
  for (int i = 0; i <= (int)(s.size()) - 2; ++i) {
    M[s.substr(i, 2)]++;
  }
  for (auto it : M) {
    if (it.second > mx) {
      mx = it.second;
      ans = it.first;
    }
  }
  cout << ans;
  return 0;
}
