CF1027F Session in BSU

Polycarp studies in Berland State University. Soon he will have to take his exam. He has to pass exactly $ n $ exams. For the each exam $ i $ there are known two days: $ a_i $ — day of the first opportunity to pass the exam, $ b_i $ — day of the second opportunity to pass the exam ( $ a_i < b_i $ ). Polycarp can pass at most one exam during each day. For each exam Polycarp chooses by himself which day he will pass this exam. He has to pass all the $ n $ exams. Polycarp wants to pass all the exams as soon as possible. Print the minimum index of day by which Polycarp can pass all the $ n $ exams, or print -1 if he cannot pass all the exams at all.

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