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