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?

The first line contains an integer $n$ $(2<=n<=10^{5})$ . The second line contains $n$ integer numbers $a_{1},a_{2},...,a_{n}$ $(1<=a_{i}<=10^{5})$ .

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