Can I enter the USA, visit Canada return to the US when the whole trip is more than 90 days?

I'm a Finnish citizen and, first, I will spend 10 days in the US, visiting my sister. I'll fly to Canada for my exchange studies and be there for 3.5 months (I have an ETa).
I have a flight back to the US to spend Christmas and New Year with my sister and her family.
My flight home to Finland in on 3rd January and my trip began on 18 August.
Am I able to reenter the US after exchange studies in Canada, or do I have to book new flights directly from there to Finland?
Best Answer
Yes, this is fine. Your initial 90-day period of admission will expire 80 days after you enter Canada, so when you return to the US nearly a month after that, you'll be given a new 90-day period of admission.
The regulation that is designed to prevent people from using Canada, Mexico, and the Caribbean for VWP resets provides that an officer may readmit the traveler for the balance of an existing 90-day period of admission. Since there is no such balance in your case, the regulation does not apply to you.
(b) Readmission after departure to contiguous territory or adjacent island. An alien admitted to the United States under this part may be readmitted to the United States after a departure to foreign contiguous territory or adjacent island for the balance of his or her original Visa Waiver Pilot Program admission period if he or she is otherwise admissible and meets all the conditions of this part with the exception of arrival on a signatory carrier.
Source: 8 CFR 217.3
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