#include<bits/stdc++.h>
using namespace std;
int main(){
	int a;
	float n;
	cin>>a;
	if(a<11) n=2.5;
	else n=2.5+(a-10)*1.5;
	printf("%.2f",n);
	return 0;
}

/**************************************************************
	Problem: 1043
	User: tangkaixuan
	Language: C++
	Result: Accepted
	Time:15 ms
	Memory:2072 kb
****************************************************************/