#include <bits/stdc++.h>
using namespace std;
int main() {
  long long tc, x, y, z, b, c, d, e, k = 1, p, q = 0, r, n, m, i, j;
  long long a[4] = {1, 3, 20, 7};
  string s, t;
  vector<char> u, v;
  set<char> st;
  set<char>::iterator it;
  map<char, long long> mp;
  k = 1;
  cin >> n >> s;
  for (i = 0; i < n - 1; i++) {
    if (s[i] != s[i + 1]) {
      k = 0;
      cout << "YES" << endl;
      cout << s[i] << s[i + 1] << endl;
      break;
    }
  }
  if (k) cout << "NO" << endl;
}
