#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int size, i, j, k, l;
  char ch1, ch2;
  char res1, res2;
  int count = 0;
  int res = 0;
  cin >> size;
  string str;
  cin >> str;
  if (str.length() == 2)
    cout << str;
  else {
    for (i = 0, j = 1; i < str.length() && j < str.length(); i++, j++) {
      ch1 = str[i];
      ch2 = str[j];
      count = 0;
      for (k = 0, l = 1; k < str.length() && l < str.length(); k++, l++) {
        if (str[k] == ch1 && str[l] == ch2) count++;
      }
      if (count > res) {
        res = count;
        res1 = ch1;
        res2 = ch2;
      }
    }
    cout << res1 << res2;
  }
  return 0;
}
