#include <bits/stdc++.h>
using namespace std;
int n;
void f(int x) {
  if (x == 0) return;
  if (n % 10)
    n -= 1, f(x - 1);
  else
    n /= 10, f(x - 1);
}
int main() {
  int tt = 1;
  while (tt--) {
    int mx = 0;
    vector<string> v;
    map<string, int> freq;
    cin >> n;
    string s;
    cin >> s;
    for (int i = 0; i + 1 < n; i++) {
      string g = "";
      char c1 = s[i], c2 = s[i + 1];
      g.push_back(c1);
      g.push_back(c2);
      freq[g]++;
      mx = max(freq[g], mx);
    }
    for (auto [key, value] : freq) {
      if (value == mx) {
        cout << key << endl;
        return 0;
      }
    }
  }
  return 0;
}
