Summing Numbers contingent on text

Hi,
I am trying to come up with a formula that is contingent on another column.
I have column A that has text in some of the cells that indicates "L"
Column B has numbers in every cell.
What I am trying to get is every time there is a "L" in column A, sum the numbers in column B.

Col A Col B
L 20
5
L 10

I am looking for the total to be 30 in column B because it disregarded the blank in col A when summing.

Hopefully, I making myself clear. Thanks