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 \rightarrow an,a1,a2,...,an-1.
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 \le n \le 105). The second line contains n integer numbers a1,a2,...,an (1 \le ai \le 105).

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