#include #include

using namespace std;

string s; int k;

char calc(string s,int k){ int cnt = 1; for (int i=1;i<=s.length()-1;i++){ if (s[i] == s[i-1]) cnt++; else cnt = 1; if (cnt>=k) return (s[i]); } return '^'; }

int main(){ cin >> k >> s; if (calc(s,k)!='^') cout << calc(s,k); else cout << "No";

}