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