#include<iostream> #include<iomanip> using namespace std; int main(){ double n,y1,y2,y3; cin>>n; if(0<=n&&n<5){ y1=-n+2.5; cout<<fixed<<setprecision(3)<<y1; } if(5<=n&&n<10){ y2=2-1.5*(n-3)*(n-3); cout<<fixed<<setprecision(3)<<y2; } if(10<=n&&n<20){ y3=n/2-1.5; cout<<fixed<<setprecision(3)<<y3; } return 0; }