Hi guys, getting really frustrated with this problem

I only want to subtract A2 from A1 and put the diffrence in B2 then follow
the list down then have the total sum lost at the bottom of collumn B.

i.e
A2 from A1 = B2
A3 from A2 = B3
B4= Sum of B2 and B3


All I seem to get is

A B
1 50 -
2 49 1 (Sum(A1-A2))
3 49 Why do I get this?
4 0
5 0
6 50 Should be 1 not 50

Cheers in advance