checked boxes - need to add corresponding numbers if boxes are checked

I am working a spreadsheet that has check boxes. Still learning.

How can I get the sheet to add lines in one row if the checkbox has been checked?

So far I have this but need it to count all of the corresponding numbers from a checked boxes

=COUNTIF(A3:A51,TRUE) & "Lines completed"

Example the below would add up to 17

FALSE 4 TRUE 5 TRUE 2 TRUE 10 FALSE 4