#include <bits/stdc++.h>
const int MOD = 1e9 + 7;
const long long int INF = 2e18;
const int mxn = 2e9 + 9;
const int mxd = 2e5 + 5;
int64_t gcd(int64_t a, int64_t b) {
  if (b == 0LL) return a;
  return gcd(b, a % b);
}
int64_t lcm(int64_t a, int64_t b) { return (a / gcd(a, b) * b); }
using namespace std;
void Run_Case() {
  int n;
  cin >> n;
  string str;
  cin >> str;
  if (n == 1) {
    cout << "NO" << '\n';
    return;
  }
  for (int i = 1; i < ((long long int)(str).length()); i++) {
    if (str[i] != str[i - 1]) {
      cout << "YES" << '\n';
      cout << str[i - 1] << str[i] << '\n';
      return;
    }
  }
  cout << "NO" << '\n';
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int T;
  T = 1;
  while (T--) {
    Run_Case();
  }
  return 0;
}
