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
/**
 * @(#)straig.java
 *
 *
 * @author
 * @version 1.00 2015/11/20
 */

import java.io.*;
import java.util.*;
public class c {

    public static void main(String [] args) throws IOException {
    	BufferedReader in=new BufferedReader(new InputStreamReader(System.in));
    	in.readLine();
    	int start=-1;
    	int copies=0;
    	int copy=0;
    	int count=0;
    	boolean unsorted=false;
    	String [] sb=in.readLine().split(" ");
    	if(Integer.parseInt(sb[0]+"")<Integer.parseInt(sb[sb.length-1]+""))//now we need to check that all of th
    	{
    		copy=Integer.parseInt(sb[0]);
    		for(int i=sb.length-1;i>1;i--){

    			if(Integer.parseInt(sb[i]+"")<Integer.parseInt(sb[i-1]+"")||Integer.parseInt(sb[i]+"")<copy){
    				System.out.println("-1");
    			return;
    			}
    		}
    		System.out.println("0");
    	}
    	else {
    		if(Integer.parseInt(sb[0])==Integer.parseInt(sb[sb.length-1]))
    		{
    			copy=Integer.parseInt(sb[0]);
    			copies=2;
    		}
    		count++;
    		for(int i=sb.length-1;i>1;i--){
    			if(Integer.parseInt(sb[i]+"")>=Integer.parseInt(sb[i-1]+"")){
    			count++;
    			if(Integer.parseInt(sb[i])==copy)
    			copies++;
    		//	System.out.println("hello"+count);
    			}
    			else{
    			start=i-1;
    			break;
    			}
    		}

			//	System.out.println(start+" " +count);
    			for(int i=0;i<start;i++){

    				if(Integer.parseInt(sb[i]+"")>Integer.parseInt(sb[i+1]+""))
    				{
    					unsorted=true;
    					break;
    				}
    			}
					if(copies==sb.length)
					{
						System.out.println(0);
						return;
					}
    				if(unsorted)
    			System.out.println(-1);
    			else System.out.println(count);

    	}
    }


}