#include<bits/stdc++.h>
using namespace std;
int main(){
	int a,ans;
	cin>>a;
	if(a<=10){
		ans=2.5;
	}
	else{
		ans=2.5+(a-10)*1.5;
	}
	cout<<ans;
}

/**************************************************************
	Problem: 1043
	User: zhuangsongyu
	Language: C++
	Result: Wrong Answer
****************************************************************/