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