Подсчитать сумму четырех введенных чисел

Public Sub СуммаЧисел()
Dim a As Double
Dim b As Double
Dim c As Double
Dim d As Double
Dim f As Double
a = InputBox("введите число")
b = InputBox("введите число")
c = InputBox("введите число")
d = InputBox("введите число")
If a < 0 Then f = b + c + d
MsgBox "f=" & f
Else: If b < 0 Then f = a + c + d
MsgBox "f=" & f
Else: If c < 0 Then f = a + b + d
MsgBox "f=" & f
Else: If d < 0 Then f = a + b + c
MsgBox "f=" & f
End If
End Sub

dim a(4)

for i=1 to 4
  if a(i)>0 then f=f+a(i)
next

Public Sub Ñóììà×èñåë()
Dim a As Double
Dim b As Double
Dim c As Double
Dim d As Double
Dim f As Double
a = InputBox("ââåäèòå ÷èñëî")
b = InputBox("ââåäèòå ÷èñëî")
c = InputBox("ââåäèòå ÷èñëî")
d = InputBox("ââåäèòå ÷èñëî")
summa = 0
If a < 0 Then a1 = a Else Sum = Sum + a
If b < 0 Then b1 = b Else Sum = Sum + b
If c < 0 Then c1 = c Else Sum = Sum + c
If d < 0 Then d1 = d Else Sum = Sum + d
MsgBox Sum, , "Ñóììà ÷èñåë"
MsgBox a1 & Chr(13) & b1 & Chr(13) & c1 & Chr(13) & d1, , "Íåñóììèðîâàííûå ÷èñëà"
End Sub

Public Sub Ñóììà×èñåë()
Dim a(4) As Double
Dim b As String
For i = 1 To 4
a(i) = InputBox("ââåäèòå ÷èñëî")
If a(i) < 0 Then b = b & a(i) & Chr(13) Else f = f + a(i)
Next i
MsgBox f, , "Ñóììà ÷èñåë"
MsgBox b, , "Íåñóììèðîâàííûå ÷èñëà"
End Sub