#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const long long int LINF = 0x3f3f3f3f3f3f3f3fll;
int main() {
  int n;
  scanf("%d", &n);
  char second[n + 10];
  scanf(" %s", second);
  for (int i = 0; i < n - 1; i++)
    if (second[i] != second[i + 1]) {
      printf("YES\n%c%c\n", second[i], second[i + 1]);
      return 0;
    }
  printf("NO\n");
  return 0;
}
