n = int(input())
s = input()
ans = None
ch = s[0]
for i in range(1,n):
if s[i]!=ch:
ans = ch+s[i]
break
if ans==None:
print("NO")
else:
print("YES")
print(ans)#include <bits/stdc++.h>
#include <math.h>
using namespace std;
int main() {
int n;
string s;
cin>>n>>s;
if (n==1) {cout<<"NO\n"; return 0;}
for (int i=0 ; i<n-1 ; i++) {
if (s[i]!=s[i+1]) {
cout<<"YES\n";
cout<<s[i]<<s[i+1];
return 0;
}
}
cout<<"NO\n";
}
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