program ABC137B
    implicit none
    integer(8)::N,i
    real(8)::out=0
    real(8),allocatable,dimension(:)::A
    read(5,*)N
    allocate(A(N))
    read(5,*)(A(i),i=1,N)

    do i=1,N
        out=out+1/A(i)
    end do

    print'(f0.10)',1/out
    
end program ABC137B