Excel中怎样编写检验合计数是否正确的宏命令

例如A1和A2是数据,A3是A1和A2的和,编制宏aaa,运行宏能检验合计数的正确性:
Sub aaa()
a = 0
For i = 1 To 2
a = a + Cells(i, "A")
Next
If a = Cells(3, "A") Then
Cells(4, "A") = "求和正确!"
Else
Cells(4, "A") = "求和出错啦!"
End If

这个不用写宏命令的 在单元格里 写=sum('单元格1':'单元格2')就好了