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

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