Nothing in Common

Alice has quarreled with Berta recently. Now she doesn't want to have anything in common with her! Recently, they've received two collections of positive integers. The Alice's integers were A1, A2, ..., AN, while Berta's were B1, B2, ..., BM. Now Alice wants to throw away the minimal amount of number from her collection so that their collections would have no common numbers, i.e. there won't be