#include <bits/stdc++.h>
using namespace std;
int a;
int main(){
      cin>>a;
	  if(a>=20&&a<=30){
	  cout<<"ok";	
	  }
      else{
      cout<<"no";	
      }
	   return 0;
}

/**************************************************************
	Problem: 1302
	User: chenyichuen
	Language: C++
	Result: Wrong Answer
****************************************************************/