【cf454B】Little Pony and Sort by Shift

2014年8月2日1,1400

One day, Twilight Sparkle is interested in how to sort a sequence of integers a1, a2, …, an in non-decreasing order. Being a young unicorn, the only operation she can perform is a unit shift. That is, she can move the last element of the sequence to its beginning:

a1, a2, …, an → an, a1, a2, …, an - 1.
Help Twilight Sparkle to calculate: what is the minimum number of operations that she needs to sort the sequence?

Input

The first line contains an integer n (2 ≤ n ≤ 105). The second line contains n integer numbers a1, a2, …, an (1 ≤ ai ≤ 105).

Output

If it’s impossible to sort the sequence output -1. Otherwise output the minimum number of operations Twilight Sparkle needs to sort it.

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题解

如果没有相同元素直接sort应该就行了

这题可以发现只能是两段不降序列且后一段能拼在前一段之前才可行,于是又变成了模拟

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