#include <bits/stdc++.h>
using namespace std;
template <typename T>
T trace2(T a, T b) {
  cerr << a << " " << b << '\n';
}
template <typename T>
T trace3(T a, T b, T c) {
  cerr << a << " " << b << " " << c << '\n';
}
template <typename T>
T trace4(T a, T b, T c, T d) {
  cerr << a << " " << b << " " << c << " " << d << '\n';
}
template <typename T>
T trace5(T a, T b, T c, T d, T e) {
  cerr << a << " " << b << " " << c << " " << d << " " << e << '\n';
}
template <typename T>
T trace6(T a, T b, T c, T d, T e, T f) {
  cerr << a << " " << b << " " << c << " " << d << " " << e << " " << f << '\n';
}
void err(istream_iterator<string> it) {}
template <typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
  cerr << *it << " = " << a << '\n';
  err(++it, args...);
}
template <typename T>
T pow(T a, T b, long long int m) {
  T ans = 1;
  while (b > 0) {
    if (b % 2 == 1) ans = (ans * a) % m;
    b /= 2;
    a = (a * a) % m;
  }
  return ans % m;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  int n;
  cin >> n;
  string s;
  cin >> s;
  for (int i = 0; i + 1 < n; i++) {
    if (s[i] != s[i + 1]) {
      cout << "YES" << '\n';
      cout << s[i] << s[i + 1];
      exit(0);
    }
  }
  cout << "NO";
  return 0;
}
