#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1010;
string word;
int freq[MAXN];
int main() {
  int n;
  cin >> n;
  cin >> word;
  for (int i = 1; i < n; i++) {
    if (word[i] != word[i - 1]) {
      printf("YES\n");
      printf("%c%c\n", word[i - 1], word[i]);
      return 0;
    }
  }
  printf("NO\n");
  return 0;
}
