#include<bits/stdc++.h>
using namespace std;
int main()
{int a;
cin>>a;
if(a>30 && a<20){
     cout<<"no"<<endl;
}else  {
	 cout<<"yes"<<endl;
} 
 


return 0;
}
/**************************************************************
	Problem: 1302
	User: czr
	Language: C++
	Result: Wrong Answer
****************************************************************/