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