#include<bits/stdc++.h>
using namespace std;
int main(){
	int a;
	cin>>a;
	if(a<=10) printf("%.2f",2.5);
	else printf("%.2f",2.5+(a-10.0)*1.5); 
	return 0;
}

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