Sharepoint - How to find out the number of the week in a calculated field?

I used the following formula. Week starts on monday. Depending on your regional settings you have to replace the ; with ,

=IF(INT((StartDate-DATE(YEAR(StartDate);1;1)+(TEXT(WEEKDAY(DATE(YEAR(StartDate);1;1)-1);"d")))/7)=0;52;INT((StartDate-DATE(YEAR(StartDate);1;1)+(TEXT(WEEKDAY(DATE(YEAR(StartDate);1;1)-1);"d")))/7))