C++ :

#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
	double x,y,t;
 	cin>>x;
 	t=int(x)/5.0;
 	switch(int (t))
 	{
		case 0:y=(-x)+2.5;break;
     	 	case 1:y=2-1.5*(x-3)*(x-3);break;
     	 	case 2:
     	 	case 3:y=x/2-1.5; break;
 	}
 	cout<<setiosflags(ios::fixed)<<setprecision(3)<<y<<endl;
 	return 0;
}