#include <stdio.h>
 
int main(void) 
{
    double e;
    scanf ("%lf", &e);
    double s = 0;
    double a=0.5;
    int i=1;
    while(a>e)
       { 
       	i++;
       	s+=a;
        a=1.0/(i*(i+1.0));
       }
    printf("%0.4lf\n", s);
	return 0;
}