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