program main
  
  implicit none
  integer,allocatable :: a(:)
  integer :: i,n
  real(8) :: tmp
  
  read(*,*) n
  allocate( a(n) )
  read(*,*) a
  
  tmp = 0e0
  do i = 1, n
    tmp = tmp + 1d0 / real( a(i), kind = 8)
  end do
  
  print*, 1d0/tmp
end program main
