subtracting in excel

[Net]

everytime i enter a number in column a i want column b to subtract 1

A
2
3
4

B
1
2
3

=SUM(A2:A4000) - 1 // will only subtract 1 once.

 

[IronWing]

B1 =A1-1
B2 =A2-1
B3 =A3-1
.
.
.

Bn = SUM(B1:B(n-1))

 

[o2brew]

or =SUM(A2:A4000) - COUNT(A2:A4000)