#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  map<string, int> m;
  int n;
  cin >> n;
  cin >> s;
  map<string, int>::iterator it;
  for (int i = 0; i < n - 1; i++) {
    string temp;
    temp.push_back(s[i]);
    temp.push_back(s[i + 1]);
    it = m.find(temp);
    if (it == m.end()) {
      m.insert(make_pair(temp, 1));
    } else
      (it->second)++;
  }
  int ans = -1;
  string q;
  for (it = m.begin(); it != m.end(); it++) {
    if (ans < it->second) {
      q = it->first;
      ans = it->second;
    }
  }
  cout << q << endl;
}
