You've gotten answers how to do this. But I wouldn't use those.
If you rearrange your data (inserting/deleting columns -- adding a single
description column in the middle of the range), then the formulas will give you
an answer, but not what you want/expect.
Instead, I'd suggest you add a helper row of indicator values.
Say you insert a new row 1 (shifting everything down a row). Then put something
in row 1 of the columns that should be summed. If you used the character X, you
could use a formula like:
=sumif($1:$1,"x",2:2)
If you insert more columns that should be added, you have to remember to add the
indicator.
And if you don't like seeing row 1 with those indicators, just hide the row.