Let's do it step by step:

You want:
ie: If L20 is 25 and I4 is 18 then M4 is +7 or 7
If L4 is 20 and I4 is 14 then M4 is -6

Could we say that M4 is equal to L4 minus I4? In that case, write in cell M4
=L4-I4, and copy this cell down, the formula will adapt automatically on each
row.