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 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();
 }
  
    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];
        int count=0;
        int Check=0;
        
        for (int i = 0; i < n; i++) 
        {
            A[i]=nextInt();
            if (i > 0 && A[i-1]>A[i])
            {
                count=i;
                Check ++;
            }
        }
      
        
        if (Check==0)
        {
            System.out.println("0");
        }
        else if (Check>=2)
        {
            System.out.println("-1");
        }
        else if (Check == 1 && A[0] < A[n-1])
        {
            System.out.println("-1");
        }
        else
        {
        
            System.out.println(n-count);
        }
        
        
        
    

    }
    
}
