Trying to sum the durations for any tasks without children

I am using SUMIF to try to sum the durations of any tasks without children. I can get it to work with dummy columns but I'm stuck on trying to do it in one formula.

I am testing with a small 5 row section. This is the formula I'm using but the sum keeps coming out wrong.

=SUMIF([Task Name]18:[Task Name]22, COUNT(CHILDREN()) = 0, Duration18:Duration22)

If you look at the picture, tasks "R, S, T, U" do not have children. Because they don't have children, I would like to sum their durations. The answer should be 20 but my formula keeps saying 0.


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