Any ideas on how I can do the following?

I have a grid similar to the following:

Column A B D
Group # Totals Group 1
1 10 Group 2
1 5 Group 3
2 20
3 7
2 5

I need the groups in Column D to update as I enter the information in
Columns A & B. For example, after I have entered everything in Columns A & B
above, I should see Group 1's total of 15 (10+5), Group 2's total of 25
(20+5), and Group 3's total of 7. Should I need to add another entry of 10
for Group 1, I would need that total to increase in Column D for Group 1 by
10.

It seems I need a "CountIf" function but not sure how to apply in a formula.