#include <bits/stdc++.h>
using namespace std;
int solve(string t, string s) {
  int ans = 0;
  for (int i = 0; i < s.length() - 1; i++) {
    string temp = "";
    temp = temp + s[i];
    temp = temp + s[i + 1];
    if (temp == t) ans++;
  }
  return ans;
}
int main() {
  std::ios::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL);
  int n;
  string s;
  cin >> n >> s;
  int ans = 0;
  string an;
  for (int i = 0; i < s.length() - 1; i++) {
    string t = "";
    t = t + s[i];
    t = t + s[i + 1];
    int sl = solve(t, s);
    if (ans < sl) {
      an = t;
      ans = sl;
    }
  }
  cout << an;
  return 0;
}
