#include<bits/stdc++.h> using namespace std; int main(){ char a[2600]; int k,z=0; cin>>k>>a; int i=0; while(a[i]!='\0'){ if(a[i]==a[i-1]){ z++; if(z==k){ cout<<a[i]<<endl; return 0; } } else{ z=1; } i++; } cout<<"No"<<endl; return 0; }