How to create Date column using M language

Hi,

I have a Data Source that contains YEAR, MONTH columns but there is no DAY column.

I wanted to create a DATE column using Query Editor/M language:

1. Add new column

2.Populate it with Dates by using 1 as DAY for all the rows in the table.

End result Should be like : 2018/1/1 (Year/Month/Day)

Any help on this much appreciated, thanks in advance !