#include <bits/stdc++.h>
using namespace std;
const int maxN = 1000 + 10;
int n;
char s[maxN];
int d[256];
int main() {
  scanf("%d", &n);
  scanf("%s", s);
  for (int i = 0; i < n - 1; i++)
    if (s[i] != s[i + 1]) {
      cout << "YES" << endl;
      cout << s[i] << s[i + 1] << endl;
      return 0;
    }
  cout << "NO\n";
}
