subtract a cell when it has a number in it from a fixed numbered cell

We are doing a fun 'biggest loser' weight loss at work and I am tasked with recording the numbers.

I have recorded the initial weigh-in (B3). from C3 to the last weigh-in N3, weights are going to be recorded. I need O3 to calculate the weight loss/difference between B3 initial weigh-in and the most current/last weekly weigh-in.

Example:

B3=200  C3=194 ........... O3 needs to = 6
B3=200  C3=194  D3=192.... O3 needs to = 8

Solution 1:

Set O3 to the formula: =B3 - LOOKUP(2,1/(C3:N3<>""),C3:N3).

To extend the formula to the cells below, drag the right-bottom corner of O3 downward: