#include <bits/stdc++.h>
using namespace std;
string s;
int call(string sub) {
  int c = 0, maxi = 0;
  for (int i = 0; i < s.size() - 1; i++) {
    if (s[i] == sub[0] && s[i + 1] == sub[1]) c++;
  }
  return c;
}
int main() {
  int n, maxi = 0;
  string gram;
  cin >> n;
  cin >> s;
  for (int i = 0; i < s.length() - 1; i++) {
    string sub = s.substr(i, 2);
    int temp = call(sub);
    if (temp > maxi) {
      maxi = temp;
      gram = sub;
    }
  }
  cout << gram << endl;
}
