#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e6 + 7;
const int inf = INT_MAX;
const long long inff = 1e18;
const int mod = 1e9 + 7;
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  string s;
  cin >> 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" << endl;
}
