Codeforces Round #259 (Div. 2) B. Little Pony and Sort by Shif

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:

Help Twilight Sparkle to calculate: what is the minimum number of operations that she needs to sort the sequence?

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

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

2
2 1

1

3
1 3 2

-1

2
1 2

0

        题意：对数组排序，你只能将最后一个元素放到第一个位置。问要操作几次。如果不能这样实现排序，输出-1。

        思路：统计第n个元素比第n-1个元素小的个数。如果有两个以上，直接输出-1。如果只有一个，比较最后一个元素和第一个元素，如果最后一个元素小于第一个元素，那么可以排序，操作次数由出现a[n]

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