#include<bits/stdc++.h>
using namespace std;
int main()
{
    int k, sum = 1;
    char a;
    string s;
    cin >> k >> s;
    a = s[0];
    for(int i = 0;i < s.length();i++)
    {
        if(s[i] == a)
        {
            sum++;
        }
        else
        {
            a = s[i];
            sum = 1;
        }
        if(sum >= k)
        {
            cout << a;
            return 0;
        }
    }
    cout << "No";
    return 0;
}