#include <bits/stdc++.h>
void met(int n, char cad[]) {
  int may = 0;
  char x;
  char y;
  for (int i = 0; i < n - 1; i++) {
    int cont = 0;
    for (int j = 0; j < n - 1; j++) {
      if (cad[i] == cad[j] && cad[i + 1] == cad[j + 1]) {
        cont++;
      }
    }
    if (cont > may) {
      may = cont;
      x = cad[i];
      y = cad[i + 1];
    }
  }
  printf("%c%c", x, y);
}
int main() {
  char cad[150];
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) {
    scanf(" %c", &cad[i]);
  }
  met(n, cad);
  return 0;
}
