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 u {
	
	BufferedReader br;
	
	void solve() {
		br = new BufferedReader(new InputStreamReader(System.in));
		int n=Integer.parseInt(ns());
		int a[]=new int[n];
		int flg=0;
		for(int i=0;i<n;i++)
		{
			a[i]=Integer.parseInt(ns());
			
		}
		int frst=n+1;
		for(int i=1;i<n;i++)
		{
			if(a[i]<a[i-1])
			{
				frst=i;
				break;
			}
		}
		
			for(int i=frst+1;i<n;i++)
			{
				flg=1;
				if(a[i]<a[i-1])
				{
					flg=2;
					break;
				}
			}
			if(flg==2)
			{
				System.out.println("-1");
			}
			else if(flg==1 || frst==n-1)
			{
				if(a[n-1]<=a[0])
				{
					System.out.println(n-frst);
				}
				else
				{
					System.out.println("-1");
				}
			}
			else
			{
				System.out.println("0");
			}
		

		
		
   }
	
	StringTokenizer input;
	
	String ns() {
		if(input == null || !input.hasMoreTokens()) {
			try {
				input = new StringTokenizer(br.readLine());
			} catch(IOException e) {
				throw new InputMismatchException();
			}
		} return input.nextToken();
	}
	
	public static void main(String[] args) {
		new u().solve();
	}
}