Statement

Marco has been studying at Politecnico di Milano for many years, he already tackled many exam sessions, but this year he has lots of hard exams so he decided to organize his study plan a bit better. He wants to prepare all the N exams he has, but noticed that not all of them are equally hard, so he estimated the number of hours H_i needed for each exam. Having followed all the lessons he also noticed that it's possible that, knowing the topics of an exam, some other exams become much easier; because of that he wants to study the subjects according to some partial ordering pair of items, while a total order is defined between all pairs.. For example studying Calculus I should be studied before Calculus II and Geometry, but those two can be studied in any order. Marco is very multitask, so much that if he completed all the prerequisites of a subject he can start right away studying that subject, even if he's already studying something else! Furthermore...