program main
	implicit none
    integer n,i
    double precision,allocatable::a(:)
    double precision::s=0
    read *, n
    allocate(a(n))
    read *, (a(i), i=1,n)
    do i=1,n
    	s=s+1/(1.0*a(i))
    end do
    print '(f0.12)',1/s
end program main