#include <cstdio>
double x;
int main()
{
    scanf("%lf", &x);
    if (x <= 5) return printf("%.3lf", -x+2.5)&0;
    if (x <= 10) return printf("%.3lf", 2-1.5*(x-3)*(x-3))&0;
    return printf("%.3lf", x/2-1.5)&0;
}