To show SHARE changes vs week#1

#1
WEEK REGION PRODUCT SHARE
1 3 10 10
1 6 5 0.10
1 2 15 3
1 1 11 0.010
1 9 1 1.05

2 9 7 10
2 8 15 0.10
2 3 10 3
2 1 1 0.010
2 9 1 2.015

3 4 10 0.001
3 2 11 0.2
3 3 10 1.01
3 1 2 1.010
3 7 3 3.005

I need to create new variable using Share column wich will be equal:

IF (SAME_REGION AND SAME_PRODUCT) NEW_COLUMN=WEEK2-WEEK1. IF (REGION=3 AND PRODUCT=10) NEW_COLUMN=3-10.
IF (SAME_REGION AND SAME_PRODUCT) NEW_COLUMN=WEEK3-WEEK1. IF (REGION=3 AND PRODUCT=10) NEW_COLUMN=1.01-10.

or

IF (SAME_REGION AND SAME_PRODUCT) NEW_COLUMN=WEEK2-WEEK1. IF (REGION=3 AND PRODUCT=10) NEW_COLUMN=3-10.
IF (SAME_REGION AND SAME_PRODUCT) NEW_COLUMN=WEEK3-WEEK2. IF (REGION=3 AND PRODUCT=10) NEW_COLUMN=1.01-3.

So I need to show share changes vs week#1. Can you please advise how can I do this?