D. Flip the Cardstime limit per test2 secondsmemory limit per test256 megabytesinputstandard inputoutputstandard outputThere is a deck of n cards. The i-th card has a number a_i on the front and a number b_i on the back. Every integer between 1 and 2n appears exactly once on the cards.A deck is called sorted if the front values are in increasing order and the back values are in decreasing order. That is, if a_i< a_{i+1} and b_i> b_{i+1} for all 1\le i<n.To flip a card i means swapping the values of a_i and b_i. You must flip some subset of cards (possibly, none), then put all the cards in any order you like. What is the minimum number of cards you must flip in order to sort the deck?InputThe first line contains a single integer n (1\le n\le 2\cdot 10^5) — the number of cards.The next n lines describe the cards. The i-th of these lines contains two integers a_i, b_i (1\le a_i, b_i\le 2n). Every integer between 1 and 2n appears exactly once.OutputIf it is impossible to sort the deck, output "-1". Otherwise, output the minimum number of flips required to sort the deck.ExamplesInput
5
3 10
6 4
1 9
5 8
2 7
Output
2
Input
2
1 2
3 4
Output
-1
Input
3
1 2
3 6
4 5
Output
-1
NoteIn the first test case, we flip the cards (1, 9) and (2, 7). The deck is then ordered (3,10), (5,8), (6,4), (7,2), (9,1). It is sorted because 3<5<6<7<9 and 10>8>4>2>1.In the second test case, it is impossible to sort the deck.
5
3 10
6 4
1 9
5 8
2 7

2 
['2-sat', 'constructive algorithms', 'data structures', 'greedy', 'sortings', 'two pointers', '*2600']