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
//I AM THE CREED
/* //I AM THE CREED
/* package codechef; // don't place package name! */
import java.io.BufferedReader; 
import java.io.IOException; 
import java.io.InputStreamReader; 
import java.util.StringTokenizer; 
import java.util.*;
import java.awt.Point;
public class Main{
    static final Random random=new Random();
    public static void main(String[] args) throws IOException 
    { 
        Scanner input=new Scanner(System.in);
        while(input.hasNext()){
            int n=input.nextInt();
            Integer[] l=new Integer[n];
            int min=Integer.MAX_VALUE;
            boolean[] suffix=new boolean[n];
            for(int i=0;i<n;i++){
                l[i]=input.nextInt();
            }
            for(int i=n-1;i>=0;i--){
                if(i==n-1){
                    suffix[i]=true;
                    continue; 
                }
                if(l[i+1]>=l[i]){
                    suffix[i]=(suffix[i+1]);
                    continue;
                }
                suffix[i]=false;
            }
            for(int i=0;i<n;i++){
                if(i==n-1){
                    min=0;
                    break;
                }
                if(suffix[i]){
                    min=0;
                    break;
                }
                if(l[n-1]<=l[0] && suffix[i+1]){
                    min=Math.min(min, n-i-1);
                    break;
                }
                if(l[i+1]<l[i]){
                    break;
                }
            }
            System.out.println(min==Integer.MAX_VALUE?-1:min);
        }
        
    }
    static int gcd(int a, int b)
    {
        if (a == 0)
            return b; 
        return gcd(b % a, a); 
    }
     
    // method to return LCM of two numbers
    static int lcm(int a, int b)
    {
        return (a / gcd(a, b)) * b;
    }
     
    //Credits to SecondThread(https://codeforces.com/profile/SecondThread) for this tempelate
    static void ruffleSort(int [] a) {
		int n=a.length;//shuffle, then sort 
		for (int i=0; i<n; i++) {
			int oi=random.nextInt(n);
			int temp=a[oi];
			a[oi]=a[i]; a[i]=temp;
		}
		Arrays.sort(a);
	}
    //Credits to SecondThread(https://codeforces.com/profile/SecondThread) for this tempelate.
    static class FastScanner {
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		StringTokenizer st=new StringTokenizer("");
		String next() {
			while (!st.hasMoreTokens())
				try {
					st=new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			return st.nextToken();
		}
		
		int nextInt() {
			return Integer.parseInt(next());
		}
		int[] readArray(int n) {
			int[] a=new int[n];
			for (int i=0; i<n; i++) a[i]=nextInt();
			return a;
		}
		long nextLong() {
			return Long.parseLong(next());
		}
	}
    
    
}