Running total of period month

[Anonymous]

Im new to Power BI ,Need some assistance is finding Running total/Accumilation in power BI

April = Open# (2)+Close#(3)/2 =3.5

May= Open# (2)+Close#(4)/2 =4

Marc= Open# (2)+Close#(19)/2 =11.5

etc...

Open Count is fixed  with April and Closed # varice by period  month

Period	Open #	Close #
April	2	3	3.5
May	3	4	4
Jun	4	5	4.5
July	5	6	5
Aug	6	7	5.5
Sep	7	8	6
Oct	8	14	9
Nov	9	15	9.5
Dec	10	16	10
jan	11	17	10.5
Feb	12	18	11
Mar	13	19	11.5

Can someone help in methods in achiveing the out put