#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n;
  char a[110];
  cin >> n >> a;
  int gram[27][27] = {0};
  for (int i = 0; i < 26; ++i) {
    for (int j = 0; j < 26; ++j) {
      for (int k = 1; a[k] != '\0'; k++) {
        if (a[k - 1] - 'A' == i && a[k] - 'A' == j) {
          gram[i][j]++;
        }
      }
    }
  }
  int big_count = 0;
  int i_index = 0;
  int j_index = 0;
  for (int i = 0; i < 26; ++i) {
    for (int j = 0; j < 26; j++) {
      if (gram[i][j] > big_count) {
        big_count = gram[i][j];
        i_index = i;
        j_index = j;
      }
    }
  }
  cout << (char)(i_index + 'A') << (char)(j_index + 'A');
  return 0;
}
