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
/* 
 Link of Question : http://codeforces.com/contest/454/problem/B
 */
import java.io.*;
import java.util.*;

public class LittlePonyandSortbyShift 
{
      static StringTokenizer st;
 static BufferedReader in;
 static PrintWriter out;


 private static int nextInt() throws IOException 
 {
        return Integer.parseInt(next());
 }

  private static long nextLong() throws IOException
  {
        return Long.parseLong(next());
  }
  private static byte nextByte() throws IOException
  {
        return Byte.parseByte(next());
  }
  
   private static String next() throws IOException
 {
        while(st == null || ! st.hasMoreTokens())
        {
            st = new StringTokenizer(in.readLine());
        }
        return st.nextToken();
 }
  private static String nextLine()
        {
            String str = "";
            try
            {
                str = in.readLine();
            }
            catch (IOException e)
            {
                e.printStackTrace();
            }
            return str;
        }

  
  

    public static void main(String[] args) throws IOException
    {
         in = new BufferedReader(new InputStreamReader(System.in));
        out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));

        int n=nextInt();
        
        int a[]= new int[n];
        
        a[0]=nextInt();
        int first=a[0];
        int idex=0;
       int count=0;
        for (int i = 1; i < n; i++) {
            
            a[i]=nextInt();
            if (first>a[i])
            {
                idex=i;
                count++;
            }
             first=a[i];
        }
        
        if (count==0)
            out.println(0);
       
        else if (count > 1 || count==1 && a[0]<a[n-1] )
            out.println(-1);
        
        else
            out.println(n-idex);

out.close();
        
        
        
        

           
    }
}

    
