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

/**************************************************************
	Problem: 1302
	User: zhengzihao3
	Language: C++
	Result: Wrong Answer
****************************************************************/