#include <bits/stdc++.h>
int n;
char str[1010];
int main() {
  scanf("%d %s", &n, str);
  for (int i = 0; i < n - 1; i++) {
    if (str[i] != str[i + 1]) {
      printf("YES\n%c%c\n", str[i], str[i + 1]);
      return 0;
    }
  }
  printf("NO\n");
  return 0;
}
