#include <stdio.h>
int main ()
{
    double n,pi,sign,i;
    pi = 0.0;
    i = 1.0;
    sign = 1;
    while(scanf("%lf",&n)!= EOF)
    {
        while(1/i > n)
        {
            pi += sign * 1/i;
            i += 2;
            sign = -sign;
            
        }
        printf("%.6lf\n",pi);   
    }
    return 0;
}