#include<bits/stdc++.h>
using namespace std;
int main(){
	int a;
	cin>>a;
	if(a<=10){
		printf("%.2f",2.5);
	}else{
		printf("%.2f",(a-10)*1.5+2.5);
	}
}

/**************************************************************
	Problem: 1043
	User: linzihang
	Language: C++
	Result: Accepted
	Time:16 ms
	Memory:2072 kb
****************************************************************/