Compare vs Previous Month

[Matute]

I want to compare the following Measure vs the same measure for the previous month, where my Total depends of platform and country.

% Visitas = SUM(Visitas) / Sum(TOTAL<MonthYear>{1<platform=P(platform),country=P(country)>}Visitas)

What should be de measure that would give me: 

% Visitas - % Visitas (Previous Month)

Thanks in advance