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