过程调用问题，不知道怎么运行，求解劝

5．运行以下程序，输出的结果为__________。
Private Function fact(n As Integer) As Integer
  fact = 1
  Do While n > 0
     fact = fact * n
     n = n - 1
  Loop
End Function
Private Sub Form_Click()
  Dim sum As Integer, i As Integer
  For i = 4 To 1 Step -1
    sum = sum + fact(i)
  Next
  Print "Sum="; sum
End Sub
请高手给讲解一下，这个程序怎么运行，看了半天实在看不懂，谢谢