Maybe this...

Data Range
A
B
3
Jan
58
4
Feb
52
5
Mar
62
6
Apr
7
7
May
73
8
Jun
4
9
Jul
22
10
Aug
11
11
Sep
80
12
Oct
59
13
Nov
73
14
Dec
31
15
------
------
16
17
18
May
252


This formula entered in B18:

=SUM(B3:INDEX(B3:B14,MATCH(A18,A3:A14,0)))