Previous month Sales

Hi everyone,

I have a report with a table where I have dimension Products and I need to calculate sales from a month (the client select that month) and the sales of the previous month.

I add a filter bar where a person could filter the month. The table shows me Sales for the selected month. Now I have to calculate the sales of the previous month.

I apply the expresion: =Month(RelativeDate([Date];-31)) to get the previous month.

The problem appear when I try to get the sales of these month. I use: If [Month]=[Previous Month] Then [Sales]

But the expresion don't show anything. Someone could help me?