called arranged if the front qualities are in expanding request and the back qualities are in diminishing request. That is, if aibi+1 for all 1≤i
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deck is called arranged if the front qualities are in expanding request and the back qualities are in diminishing request. That is, if
To flip a card I implies trading the upsides of simulated intelligence and bi. You should flip some subset of cards (perhaps, none), then, at that point, put every one of the cards in any request you like. What is the base number of cards you should flip to sort the deck?
Input
The main line contains a solitary integer n (1≤n≤2⋅105) — the number of cards.
The following n lines depict the cards. The I-th of these lines contains two integers ai,bi (1≤ai,bi≤2n). Each integer somewhere in the range of 1 and 2n shows up precisely once.
Output
In case it is difficult to sort the deck, output "- 1". In any case, output the base number of flips needed to sort the deck.
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