|Just by way of a more complete explanation of why your formula is incorrect:|
Excel calculates from the "inside out",
So with your formula, the first thing it tries to calculate are the two expressions in parentheses ( ):
Your adding specific cells, if you remove one of the cells, one of the formula parameters is removed, and up pops the #REF error.
The SUM() function does not come into play until after the inner expressions have been evaluated.