#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 5;
const int K = (1 << 17) + 10;
const long long int INF = 2e18 + 500;
const int inf = 0x7fffffff;
const int SQRT = 500;
const int lgN = 20;
const int MOD = 998244353;
int main(int argc, char const *argv[]) {
  cin.sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  string s;
  cin >> s;
  for (int i = 1; i < n; i++) {
    if (s[i] != s[i - 1]) {
      cout << "YES\n";
      cout << s[i - 1] << s[i] << endl;
      exit(0);
    }
  }
  cout << "NO\n";
  return 0;
}
