Measure for 2 last months

Hello, i am new to powerBi and i have a table which has column Month in string format like this

2016-05
2016-06
2016-07
2016-08

I want to have measure which always chooses 2 top values and put it like slicer for graph.

I tried TOPN method but without luck.

For this example measure should just show

2016-07
2016-08

Thanks in advance for help