#include<cstdio>

 #include<cstring>

 #include<string>

 #include<cmath>

 #include<iostream>

 #include<algorithm>

 #include<map>

 #include<set>

 #include<queue>

 #include<vector>

 using namespace std;

 typedef long long ll;

 typedef unsigned int uint;

 typedef unsigned long long ull;

 typedef pair<int,int> PII;

 typedef vector<int> VI;

 #define fi first

 #define se second

 #define MP make_pair

 #define N   11000

 #define MOD 1000000007

 #define eps 1e-8

 #define pi acos(-1)

 int s[N][];

 char ch[N];

 int main()

 {

     int n;

     scanf("%d",&n);

     scanf("%s",ch+);

     for(int i=;i<=n;i++)

     {

         for(int j=;j<=;j++) s[i][j]=s[i-][j];

         s[i][ch[i]-'a'+]++;

     }

     int x=;

     int y=;

     for(int i=;i<=n;i++)

      for(int j=i;j<=n;j++)

       if(x+y==)

       {

              int len=j-i+;

              int tmp=;

              for(int k=;k<=;k++) tmp=max(tmp,s[j][k]-s[i-][k]);

              if(tmp<=len/){x=i; y=j;}

       }

     if(x+y>)

     {

         printf("YES\n");

         for(int i=x;i<=y;i++) printf("%c",ch[i]);

     }

      else printf("NO\n");

     return ;

 }