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

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