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.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;


import java.util.StringTokenizer;


public class MainClass {

    static class FastReader
    {
        BufferedReader br;
        StringTokenizer st;
        public FastReader()
        {
            br = new BufferedReader(new
                    InputStreamReader(System.in));
        }
        String next()
        {
            while (st == null || !st.hasMoreElements())
            {
                try
                {
                    st = new StringTokenizer(br.readLine());
                }
                catch (IOException  e)
                {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }
        int nextInt()
        {
            return Integer.parseInt(next());
        }
        long nextLong()
        {
            return Long.parseLong(next());
        }
        double nextDouble()
        {
            return Double.parseDouble(next());
        }
        String nextLine()
        {
            String str = "";
            try
            {
                str = br.readLine();
            }
            catch (IOException e)
            {
                e.printStackTrace();
            }
            return str;
        }
    }

    public static void main(String[] args) {
        FastReader in = new FastReader();
        int n = in.nextInt() ;
        int [] arr = new int[n] ;
        int [] dp = new int[n-1] ;
        for (int i = 0 ; i < n ; i++)
        {
            arr[i] = in.nextInt() ;
        }
        for (int i = 0 ; i < n-1 ; i++)
        {
            dp[i] = arr[i]-arr[i+1] ;
        }
        int countPositiveIndex = 0 , index = -1;
        for (int i = 0 ; i < n-1 ; i++ )
        {
            if (dp[i] > 0)
            {
                countPositiveIndex++ ;
                index = i+1 ;
            }
        }
        if (countPositiveIndex == 1)
        {
            if (arr[n-1] <= arr[0])
                System.out.println(n-index);
            else
                System.out.println(-1);
        }
        else if (countPositiveIndex == 0)
        {
            System.out.println(0);
        }
        else
            System.out.println(-1);


    }
}