#include<bits/stdc++.h>
using namespace std;
int main()
{
	int a;
	cin>>a;
	if(a>=0&&a<=5)
	cout<<0-a+2.5<<endl;
	if(a>=5&&a<=10)
	cout<<2-1.5*(a-3)*(a-3)<<endl;
	if(a>=10&&a<=15)
	cout<<a/2-1.5<<endl;
	return 0;
}