#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: chensijia
	Language: C++
	Result: Wrong Answer
****************************************************************/