#include<bits/stdc++.h>
using namespace std;
double x,y;
int main(){
    cin>>x;
    if(x<5)
        y=2.5-x;
    else if(x<10)
        y=2-1.5*(x-3)*(x-3);
    else
        y=x/2-1.5;
    cout<<fixed<<setprecision(3)<<y;
    return 0;
}