#include <bits/stdc++.h>
using namespace std;
int main(){
	float f;
	cin>>f;
	if(f<=10)cout<<"2.50";
	else printf("%.2f",2.5+(f-10)*1.5);
	return 0;
}

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