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.math.BigInteger;
import java.util.*;
 
public class Minimise {
    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 reverse(int[] a, int li, int ri){
    while(li < ri){
      int temp = a[li];
      a[li]= a[ri];
      a[ri] = temp;
      
      li++;
      ri--;
    }
  }

  public static void rotate(int[] a, int k){
    k = k % a.length;
    if(k < 0){
      k += a.length;
    }

    reverse(a, 0, a.length - k - 1);
    reverse(a, a.length - k, a.length - 1);
    reverse(a, 0, a.length - 1);
  }

    
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
	
		FastReader sc = new FastReader();
    	
        int n = sc.nextInt();
        int[] arr=new int[n];
        int k=0;
        int max=0;
        for(int i=0;i<n;i++)
        {
            arr[i]=sc.nextInt();
            max= Math.max(max,arr[i]);
        }
        for(int i=n-1;i>0;i--)
        {
            if(arr[i]<arr[i-1])
            {
                k=n-i;
                break;
            }
        }
        if(k==0)
        {
            System.out.println(0);
        }
       else
       { int count=0;
           rotate(arr,k);
           for(int i=0;i<n-1;i++)
           {
               if(arr[i]>arr[i+1])
               {
                   System.out.println(-1);
                   count++;
                   break;
               }
           }
           if(count==0)
           {
               System.out.println(k);
           }
       }}}