Subtotalling variable data

Hi there. In the attached sheet I am trying to return a Total in cell H5 (sheet 2) based on the following steps:
Calculate no. of units in sheet 1, column L where ("£1.25" = 1 unit and "£2.50" = 2 units)
Subtract units where there is a value in sheet 1, column M greater than zero. For example, row 13 in sheet 1 would total 1 unit ("£1.25" in L13 = 1 unit and there's nothing above zero in M13); row 20 would total 2 units using the same logic; but row 35 would be zero units as M35 = >0.
Add the whole lot together to return my Total in sheet 2, H5.

The problem I'm having is with getting it to subtract the appropriate number of units when it sees a value above zero in column M - i.e. subtract 1 unit when L*=£1.25 but subtract 2 units when L*=£2.50.

Any help would be appreciated.

Thanks