It's a good idea to be explicit with parentheses when building arithmetic expressions.  For example, at first the following code doesn't look like it would cause a problem:

Dim i As Integer = 1
i = i + i - i
Console.Writeline(i)

Executing this code will return 1 as you should expect.  But consider what happens in this case:

Dim i As Integer = i.MaxValue
i = i + i - i
Console.Writeline(i)

This code ends with an overflow error which could have been prevented by simply using parentheses like this:

i = i + (i - i)