#include<bits/stdc++.h> using namespace std; int main(){ double n,y; cin>>n; if(1<=n && n<5){ y = 2.5-n; }else if(5<=n && n<10){ y = 2-1.5*(n-3)*2; }else if(10<=n && n<20){ y = n/2-1.5; } printf("%.3f",y); return 0; }