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