# include<iostream> # include<cstdio> # include<iomanip> using namespace std; int main(){ double y=0,x; cin>>x; if(0<=x&&x<5){ y=(x-x-x)+2.5; } if(5<=x&&x<10){ y=2-1.5*(x-3)*2; } if(10<=x&&x<20){ y=x/2-1.5; } printf("%.3lf",y); return 0; }