#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int n;
string s;
void test_case() {
  cin >> n;
  cin.ignore(110, '\n');
  cin >> s;
  string ans;
  int mx_cnt = -1;
  for (int i = 0; i < n - 1; ++i) {
    string sub = s.substr(i, 2);
    int cnt = 0;
    for (int j = 0; j < n - 1; ++j) {
      if (sub[0] == s[j] && sub[1] == s[j + 1]) {
        ++cnt;
      }
    }
    if (cnt > mx_cnt) {
      mx_cnt = cnt;
      ans = sub;
    }
  }
  cout << ans << endl;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  test_case();
  return 0;
}
