convert week to Month

Dear gurus,

I have a year column (with only 2017) and week column. The data doesnt have date column. However, I need to convert weeks to respective months. How should I do it in ALteryx? Moreover, 

week 5 is Jan 30 to Feb 5. So how should I convert the week number to two months?

Year     Week    Sales

2017       5          200

2017       5          100

2017       6          300

2017       9          400

Your help in this matter will be sincerely appreciated.