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