#include <bits/stdc++.h>
using namespace std;
const long long maxn = 200000 + 5;
long long p[maxn], h[maxn];
std::vector<long long> g[maxn];
bool flag;
pair<long long, long long> dfs(long long u, long long par) {
  pair<long long, long long> currNode = {0, 0};
  for (long long v : g[u]) {
    if (v == par) continue;
    pair<long long, long long> x = dfs(v, u);
    currNode.first += x.first;
    currNode.second += x.second;
  }
  long long x = p[u] + h[u] + currNode.first + currNode.second;
  if (x & 1) flag = false;
  x /= 2;
  if (x < 0 or x > currNode.first + currNode.second + p[u]) flag = false;
  if (currNode.first > x) flag = false;
  return {x, currNode.first + currNode.second + p[u] - x};
}
void solve() {
  int n;
  cin >> n;
  string s;
  cin >> s;
  if (n == 1) {
    cout << "NO" << endl;
    return;
  }
  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;
    }
  }
  cout << "NO" << endl;
  return;
}
int main() {
  int t = 1;
  for (int q = 1; q <= t; q++) {
    solve();
  }
  return 0;
}
