Month to date total

[Wattsie_100]

I have a data set as follows:
SaleAmt Date
10 01JUN2016
20 02JUN2016
15 03JUN2016

I want to make create a new column with the month to date total which would look like:
MonthToDate
10
30
45

I believe I need to use the retain statement but I am totally stumped!

[Reeza]

Yes, you need the RETAIN statement. However, the following also includes an implicit retain. 

Running_total + SaleAmt;