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