vb 如何把两个运算结果相加

我用了两个inputbox输入两个值相乘，而且做循环，我想把每个算出来的值都加在一起怎么办？把刚加上去的再减下来又该怎么办就像返回上一步一样？

Dim sum As Double
Dim a As String
Dim b As String
Dim p As String
sum=0.0
Do
 a=InputBox("a","Input","")
 b=InputBox("b","Input","")
 if a="" and b="" Then Exit Do
 sum=sum+CDbl(a)*CDbl(b)
 Debug.Print "sum:";sum
 p=InputBox("c/b to Continue/Back:“,"Input","c")
 if p=="b" then 
  sub=sum-CDbl(a)*CDbl(b)
  Debug.Print "sum:";sum
 endif
Loop