#include<iomanip>
using namespace std;
int main()
{
	double w,h;
	cin>>w;
	if(0<=w&&w<5)
	{
	h=-w+2.5;
	}
	else if(5<=w&&w<10)
	{
	h=2-1.5*(w-3)*(w-3);
	}
	else if(10<=w&&w<20)
	{
	h=w/2-1.5;
	}
	cout<<fixed<<setprecision(3)<<h;
}