Excel: check text in cell, add values

Hello,

I'm trying to create a formula to check a cell for a certain string of text, and then if it contains it, take the value of a number in that row and and then add them all together.
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So for example:
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A1 - A Dog
A2 - The Dog
A3 - A Cat
A4 - A Bird
A5 - The Dog
....
B1 - 10
B2 - 5
B3 - 10
B4 - 10
B5 - 5
...
So I'd like to check the cells in column A, see if it contains the word "dog" and if it does, take the value in column B and sum them all together; so in this case, the subsequent value left in that cell would be 20


Thanks for your help in advanced.