Running total formula

My cells look like this.
in a:4 value is 6. In b:4 the value is 112.
in a:5 value is 7. In b:5 the value is 125.
in a:6 value is 6. in b:6 the value is 110.
in a:7 value is 7. in b:7 the value is 126.
What I want to do is have the differences in the c:4 thru c:7.
would be b:7-b:5 . 1 would be the result in c:5.
then the result of b:6-b:4. the result would be minus 2.
I am working with the numbers 4,5,6and 7 in the a: cell range. that range
is constant. the b range values change. To keep the totals runing I want to
use the c:range for the 4's, d:range for the 5's, e:range for the 6's and
f:range for the 7's. What formula do I need to put in the c,d,e,and f ranges
to do this.
Thank you.
always dazed and confused Dave.