How about ...

A
B
C
D
E
1
Month
Accts Added
Accts Deleted
Net Accts
2
 May
23
8
15
 D2: =B2-C2+N(D1)
3
 June
47
18
44
4
 July
16
1
59
5
 August
12
29
42
6
 September
30
9
63
7
 October
45
17
91
8
 November
18
5
104