#include<bits/stdc++.h> using namespace std; int main(){ char a[2550]; int k,t=1; cin>>k; cin>>a+1; int len=strlen(a+1); for(int i=1;i<=len;i++){ if(t>=k){ cout<<a[i]; return 0; } if(a[i]==a[i+1]) t++; else t=1; } cout<<"No"; return 0; }