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#F1951. Little Pony and Sort by Shift

    ID: 1957 传统题 2000ms 256MiB 尝试: 0 已通过: 0 难度: 2 上传者: 标签>模拟编程与模拟基础难度共享题库Codeforces英文题面题目来源题面语言

Little Pony and Sort by Shift

题目描述

B. Little Pony and Sort by Shift
time limit per test
1 second
memory limit per test
256 megabytes
input
standard input
output
standard output

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.

Examples
Input
2
2 1
Output
1
Input
3
1 3 2
Output
-1
Input
2
1 2
Output
0

题目来源：fps-www.educg.net-codeforce-1-2833.xml.zip；FPS 共享题包，题包内第 1531 题。保留原作者与原赛事署名。