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
import java.io.*;
import java.util.*;

public class pony_sort_by_shift 
{
	boolean checkSorted(int [] a)
	{
		for (int i = 1; i< a.length; i++)
			if (a[i]<a[i-1])
				return false;
		
		return true;
	}
	
	int isPossible(int [] a, int minind)
	{
		int n = a.length;
		int [] newa = new int [n];
		for (int i = 0; i<= minind-1; i++)
			newa[n-minind+i] = a[i];
		for (int i = minind ; i< n; i++)
			newa[i-minind] = a[i];
		
		if (checkSorted(newa))
			return n-minind;
		else return -1;
	}
	
	public static void main(String args[])throws IOException
	{
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		PrintWriter out = new PrintWriter(System.out);
		pony_sort_by_shift ts = new pony_sort_by_shift();
		
		int n = Integer.parseInt(br.readLine());
		int [] a = new int [100000+9];
		String [] tokens = br.readLine().split(" ");
		for (int i = 0; i< n; i++)
			a[i] = Integer.parseInt(tokens[i]);
		int i = 0;
		for (; i<n-1; i++)
		{
			if (a[i] > a[i+1])
				break;
		}
		if (i == n-1)
			{System.out.println(0); return;}
		int j = i+1;
		for (; j< n-1; j++)
		{
			if (a[j] > a[j+1])
				break;
		}
		if (j != n-1     ||  a[n-1] > a[0])
		{System.out.println(-1); return ;}
		else
			System.out.println(n-i-1);
	
		
		
		out.close();
		out.flush();
	}

}
