Comparing first two weeks to last two weeks

[amaanebrahim]

I have weekly data. I would like to create a monthly indicator showing the % change from the last two weeks compared to the last two weeks for each month.

The output should look as follows:
A B
1 16M01 % (where this calculation is ((16W03+16W04)-(16W01+16W02)) / (16W01+16W02) * 100
2 16M02 % (where this calculation is ((16W07+16W08)-(16W05+16W06)) / (16W05+16W06) * 100

Thanks all your help in advance

Example Data

16W01	102080000​
16W02	144580000​
16W03	143400000​
16W04	136760000​
16W05	143380000​
16W06	134140000​
16W07	133580000​
16W08	137140000​