Change the sign of a number from positive to negative

Hello SCN experts,

I am working on a report that has 2 movements types: Entry of qty (101) and reverse of that qty (102).

However both quantities are positive numbers. Movement type 102 should reverse the qty of movement type 101.

This is a production report and we need to sum the total quantity under movement type 101 and exclude the mov type 102.

How can I create a variable to change the sign of the qty under mov type 102 from positive to negative?

Thank you,