#include<bits/stdc++.h> using namespace std; int main(){ double n,y; 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; }