#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int n, mx = -1;
  cin >> n;
  string s, sb = "";
  cin >> s;
  map<string, long long int> mp;
  for (long long int i = 0; i < n - 1; i++) {
    string sa = "";
    for (long long int j = 0; j < 2; j++) {
      sa += s[i + j];
    }
    mp[sa]++;
  }
  for (auto it = mp.begin(); it != mp.end(); it++) {
    if (mx < it->second) {
      mx = it->second;
      sb = it->first;
    }
  }
  cout << sb;
}
