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.*;
import java.math.*;

public class B{
    public static void main(String args[])throws IOException{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));
        Scanner s = new Scanner(System.in);
        
        int n = Integer.parseInt(br.readLine());
        int[] ar = new int[n];
        String[] inp = br.readLine().split(" ");
        for(int i=0; i<n; i++)
            ar[i] = Integer.parseInt(inp[i]);
        
        int posL = -1, posR = -1;
        for(int i=1; i<n; i++){
            if(ar[i] < ar[i-1]){
                posL = i;
                break;
            }
        }       
        
        for(int i=n-2; i>=0; i--){
            if(ar[i] > ar[i+1]){
                posR = i+1;
                break;
            }
        }
        
        //System.out.println(posL+" "+ posR);
        if(posL == -1 && posR == -1)
            bw.append("0\n");
        else{
            if(posL == posR){
                if(ar[0] >= ar[n-1])
                    bw.append((n-posR) + "\n");
                else
                    bw.append("-1\n");  
            }
            else
                bw.append("-1\n");
        }
                
        bw.flush();
        bw.close();
        
    }
}