/*想啥呢，肯定没有



























































































































*/

还看！没有！

#include <iostream>
#include <string>
using namespace std;

int main() {
    int k;
    string s;
    cin >> k >> s;
    
    int n = s.length();
    if (k > n) {
        cout << "No" << endl;
        return 0;
    }
    
    for (int i = 0; i <= n - k; i++) {
        char c = s[i];
        bool found = true;
        for (int j = 1; j < k; j++) {
            if (s[i + j] != c) {
                found = false;
                break;
            }
        }
        if (found) {
            cout << c << endl;
            return 0;
        }
    }
    
    cout << "No" << endl;
    return 0;
}

乌拉！