#include<bits/stdc++.h>
using namespace std;
int main()
{
	
	int a;
	cin>>a;
	if(a>=20 && a<=30){
		cout<<"ok"<<endl;
	}else{
		cout<<"no"<<endl;
	}
	
		return 0;
	
}
/**************************************************************
	Problem: 1302
	User: ct001
	Language: C++
	Result: Wrong Answer
****************************************************************/