#include<iostream> using namespace std; int main(){ double x,y; cin>>x; if(0<=x && x<5){ y = -1*x+2.5; } else if(5<=x && x<10){ y = 2-1.5*(x-3)*2; } else if(10<=x && x<20){ y = x/2-1.5; } printf("%.3lf",y); return 0; }