Hello everyone.
I have a base that contains sales information per store, product and month, as shown below:

I need to pick up every seven months and create a period column (Jan16-Jul16 / Feb16-Aug16) by grouping sales together:

the sum is made as follows:
- period Jan16-Jul16 was the sale of all the months of this period. In this case I had one sale in July, so the value will be 1.
- period Feb16-Aug16 was the sale of all the months of this period. In this case I had one sale in July and two in August, then the value will be 3.
and so goes for the whole base.
Can someone help me?