program resistors_in_parallel
  implicit none
  integer :: n
  real(8) :: a(100) = 0.d0
  character(64) :: s
  read(*,*) n
  read(*,*) a(1:n)
  write(s,'(f32.16)') 1.d0/sum(1.d0/a(1:n))
  write(*,'(a)') trim(adjustl(s))
end program resistors_in_parallel