Defining a range for SUM based on a variable value in another cell

I have explored tons of forums and have found similar situations but no solution.

I have a function in AB3 that counts units sold. Then in cell AB5, I am trying to incorporate a SUM function that only SUMS as many cells as the value that is in AB3.

Example, I sell 107 units in January, I want to SUM the values in Z2:Z108. If I sell 10 units in December then I want to SUM the values in Z2:Z11.

The last formula that I used was =SUM(OFFSET(Z2:Z999,AB3,0)) but it returned the wrong SUM. I can't tell exactly what it's grabbing. I assume I've used this formula incorrectly.

Please help!

That worked perfect. Thanks!