#include<bits/stdc++.h>
using namespace std;
int main(){
	int t;
	cin>>t;
	if(20<=t&&t<=30){
		cout<<"OK"; 
	}else cout<<"NO";
	return 0;
}
/**************************************************************
	Problem: 1302
	User: panjinyan
	Language: C++
	Result: Accepted
	Time:10 ms
	Memory:2072 kb
****************************************************************/