What I am wanting to do is show in one cell the difference in values from week to week. What I am doing is tracking weight loss totals and we are wanting to enter the current weight for the week and calculate the difference to the previous week; this is over 10 week period. 

Example: Week 1 = B, Week 2 = C, Week 3 = D, Week 4 = E
This goes on for 10 total weeks.

When B is entered, nothing should happen; when C is entered it should calculate the difference from B; when D is entered it would calculate the difference from C. The thing is, I would want this to change only for the one cell that has the calculation.

These values are in a row. Week 1 is column B, week 2 is column C and so on. The calculation would be in column O.

Thanks for the help!!!
Regards,
J