Janice is choosing some CDs from her collection to take to a party. She owns 10 jazz CDs, 15 pop CDs and 8 hip hop CDs.

Fred is bringing 6 pop CDs to the party. Janice knows that Fred owns 4 of the pop CDs that are in her collection. How many pop CDs must she bring to the party to ensure that between them they have brought at least 9distinct pop CDs?

I thought about the pigeonhole principle,but it turn out the answer is 20766240. Can someone pls help