Need help with template please

I'm coming up with a form for tracking our high school reunion totals ,,
i hope i can write this so it makes sense.

We're hoping to have about 1500 people ,,and i have totals for pub nite
costs in tab M and open house costs in tab N ,, i would like to have this
total in Tab O.

Whew okay ,, now i would like the pub nite,Tab M,to total up around the
1500 mark and open house,Tab N,to total up at 1500 as well and then a
grand total from Tab O .

Can this be done and how.

Thank you ,,,and my hair thanks you!

Gord