Statement

Everyone knows dating is hard. Especially if people spend most of their free time reading books! On the occasion of Valentine's Day the book club Dead Authors Society tries to help the issue with their newly started Cupido Program. Each applicant to the program must provide their library ID number (ID_i) and the library IDs of up to 3 other program participants whom they fancy (A_i, B_i, C_i). If an applicant left a field in the application form empty it is denoted by the number -1. Let's find the number of mutual sympathies among the program participants! Two persons, i and j form a mutual sympathy if person i listed the ID of person j, and person j listed the ID of person i among the participants whom they fancy.