Feed them ORANGE

I have a truck full of $200$ oranges. I want you to divide them among 13 girls and 5 boys, so that each girl gets $m$ orange(/s) and each boy gets $n$ orange(/s).

Consider, $a=m+n$ ;

What is the sum of all possible values of $a$ ?

Details and assumptions

$m$ and $n$ must be non-zero positive integers.

All 200 oranges must be distributed.