#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pll = pair<ll, ll>;
using pii = pair<int, int>;
using vi = vector<int>;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  int n;
  string s;
  cin >> n >> s;
  for (int i = 0; i < n - 1; i++) {
    if (s[i] != s[i + 1]) {
      cout << "YES" << '\n';
      cout << s[i] << s[i + 1] << '\n';
      return 0;
    }
  }
  cout << "NO" << '\n';
}
