Merging two rows and averaging the columns of those rows in pandas

Question

How can I merge/combine two rows with the same ids (row 1 and 2) and average out columns A and B. So that my dataframe would go from this:

   id  A   B
0  22  25  3
1  23  24  5 <-
2  23  56  3 <-
3  24  34  6

to this:

   id  A   B
0  22  25  3
1  23  40  4 <- averages of the two rows for A and B
2  24  34  6

Basically two rows are collapsed into one. The data in column A is averaged with column A, and column B is averaged with column B.