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