[Python] How do I sum values in a column that match a given condition using pandas?

Suppose I have a dataframe like so:

a b
1 5
1 7
2 3
1 3
2 5


I want to sum up the values for b where a = 1, for example. This would give me 5 + 7 + 3 = 15.

How do I do this in pandas?

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