#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  string a;
  cin >> n >> a;
  string tp1, tp2, ans;
  int mx = 0;
  int tn = 0;
  for (int i = 0; i < n - 2; i++) {
    tp1.clear();
    tn = 0;
    tp1.push_back(a[i]);
    tp1.push_back(a[i + 1]);
    for (int j = 0; j < n - 1; j++) {
      if (a[j] == tp1[0] && a[j + 1] == tp1[1]) {
        tn++;
      }
    }
    if (tn > mx) {
      mx = tn;
      ans = tp1;
    }
  }
  if (n == 2) {
    ans = a;
  }
  cout << ans << endl;
  return 0;
}
