#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; }