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