Complicated IF formula

Hi all,

I hope to find some help in preparing my dataset for analyses. To save me from a looot of time-consuming manual work, I need a formula for the following:

I have the following variables:
week no.
week day
tv contacts
purchase

And I want to compute the new variable:
tv contacts before a purchase in that week

This new variable should give the number of tv contacts of that day, IF there has not (yet) been a purchase in that week, until that day.
(If the tv contacts are on the same day as the purchase, the formula should give the number of tv contacts.)

I hope the Excel sheet I attached clarifies. Looking forward to your responses.

Kind regards,
M