#include<iostream>
#include<iomanip>
using namespace std;
int main(){
	int a;
	double x,y;
	cin>>x;
	if(0<=x&&x<5)
	a=1;
	else if(5<=x&&x<10)
	a=2;
	else if(10<=x&&x<20)
	a=3;
	switch(a){
		case 1:
			y=-x+2.5;
			cout<<fixed<<setprecision(3)<<y;break;
		case 2:
			y=2-1.5*(x-3)*(x-3);
			cout<<fixed<<setprecision(3)<<y;break;
		case 3:
			y=x/2.0-1.5;
			cout<<fixed<<setprecision(3)<<y;break;
	}
	return 0;	
}