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