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