CF454B.Little Pony and Sort by Shift

One day, Twilight Sparkle is interested in how to sort a sequence of integers _a_1, _a_2, ..., a__n 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:

_a_1, _a_2, ..., a__n → a__n, _a_1, _a_2, ..., a__n - 1.

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

一天，暮光闪闪对如何将一串整数 $a_1, a_2, \dots, a_n$ 按非递减顺序排序产生了兴趣。作为一名年轻的独角兽，她唯一能执行的操作是单位移位：即将序列的最后一个元素移动到最前面：

$$a_1, a_2, \dots, a_n \rightarrow a_n, a_1, a_2, \dots, a_{n-1}.$$

请帮助暮光闪闪计算：她需要执行的最少操作次数是多少？

The first line contains an integer n (2 ≤ n ≤ 105). The second line contains n integer numbers _a_1, _a_2, ..., a__n (1 ≤ a__i ≤ 105).

第一行包含一个整数 $n$（$2 \leq n \leq 10^5$）。第二行包含 $n$ 个整数 $a_1, a_2, ..., a_n$（$1 \leq a_i \leq 10^5$）。

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

如果无法将序列排序，则输出 $-1$。否则，输出暮光闪闪将其排序所需的最少操作次数。