Sum values by 2 weeks a time

Hi guys, i have the following problem:

the column's label is the number of the week in the year. Now i would like to sum values taking in account two consecutive weeks a time. For example i want to sum values of the week 17 + values of the week 18 and so for each couple of week ( In this way i get a table with 26 columns instead of 52).

Thank you for any suggestion .