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.io.PrintWriter;
import java.io.StreamTokenizer;
import java.util.Scanner;

public class Temp {
	static BufferedReader bf;
	static PrintWriter pw;

	public void Solve() {

	}

	public static void main(String args[]) throws IOException {
		bf = new BufferedReader(new InputStreamReader(System.in));
		pw = new PrintWriter(System.out);
		StreamTokenizer st = new StreamTokenizer(bf);
		st.nextToken();
		int n = (int) st.nval;
		int[] list = new int[n];
		boolean can = true;
		boolean founddec = false;
		int pos = -1;
		for (int i = 0; i < n; i++) {
			st.nextToken();
			list[i] = (int) st.nval;
		}

		for (int i = 1; i < n; i++) {
			if (list[i] < list[i - 1]) {
				if (founddec) {
					can = false;
					break;
				} else {
					founddec = true;
					pos = i;
					if (list[n - 1] > list[0]) {
						can = false;
						break;
					}
				}
			}
		}

		if (can) {
			if (founddec)
				System.out.println(n - pos);
			else
				System.out.println(0);
		} else
			System.out.println(-1);

		bf.close();
		pw.close();

	}

}
