简单！！！

#include <bits/stdc++.h>

using namespace std;

int main() {

int k;

string s;

cin >> k >> s;

if (k == 1 && !s.empty())  {

    cout << s[0] << endl;
    
    return 0;
    
}
int count = 1;

for (int i = 1; i < s.size();  ++i) {

    if(s[i] == s[i-1]) {
    
        count++;
        
        if (count >= k) {
        
            cout << s[i] << endl;
            
            return 0;
            
        }
        
    }else {
    
        count = 1;
        
    }
    
}
cout << "No" << endl;

return 0;

}