Sumif help

Hi
I'm looking for a formula to add number in a column based on a number in a different column in the same row.

Example:

A B
250 5
100 1
250 2
300 1
100 3
100 3
100 4



Basically what I want to do is if Column A has the value"100" I want to add the number in column B and put the sum in cell D5 for example. In this case the value in cell d5 should be "11"