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

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