If a + b + ab = 2019, where a,b are positive integers, find the sum of all possible values of a ?

Answer 1

-2021 -2
-1011 -3
-506 -5
-405 -6
-203 -11
-102 -21

0 2019
1 1009
3 504
4 403
9 201
19 100

36 is the sum of all the possible positive values of 'a' that satisfies the above equation.