#include <bits/stdc++.h>
using namespace std;
char a[1000 + 5];
int main() {
  int n;
  scanf("%d", &n);
  getchar();
  scanf("%s", a);
  for (int i = 1; i < n; i++) {
    if (a[i] != a[i - 1]) {
      printf("YES\n%c%c\n", a[i - 1], a[i]);
      return 0;
    }
  }
  printf("NO\n");
  return 0;
}
