#include<bits/stdc++.h>    
using namespace std;    
  
int main()  
{  
    string a;  
    int k;  
    cin >> k >> a;  
    bool judge=true;  
    char imp = ' ';  
      
    for(int i=k-1;i<a.length();i++){  
        for(int j=0;j<k-1;j++){  
            if(a[i-j]!=a[i-j-1]){  
                judge = false;  
                break;  
            }  
        }  
        if(judge){  
            imp = a[i];  
            break;  
        }  
        else judge= true;   
    }  
      
    if(imp==' ') cout << 'No';  
    else cout << imp;  
    return 0;  
}