Summing cells with different numbers of rows (large data sheet, HELP)

Hello,

I have a large amount of data, and I need to figure out a formula to sum up only the values with specific numeric labels. Here's an example of the layout:

1 32
1 45
1 11
1 38
1 65
2 45
2 89
2 67
3 55
3 45
3 67
3 10

As you can see, each label in the first column has a different number of rows. How could I insert a formula to the right of this that would only sum the values next to 1's, 2's 3's, and so on?

Thanks,
B