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.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.HashSet;
import java.util.Locale;
import java.util.StringTokenizer;

public class B_259 {
	static StringTokenizer st;
	static BufferedReader br;
	static PrintWriter pw;

	public static void main(String[] args) throws IOException {
		Locale.setDefault(Locale.US);
		br = new BufferedReader(new InputStreamReader(System.in));
		pw = new PrintWriter(new BufferedWriter(new OutputStreamWriter(
				System.out)));
		int n = nextInt();
		Integer[] a = new Integer[n + 2];
		HashSet<Integer> h = new HashSet<Integer>();
		for (int i = 1; i <= n; i++) {
			a[i] = nextInt();
			h.add(a[i]);
		}
		if (h.size() == 1) {
			System.out.println(0);
			return;
		}
		int ind = -1, k = 2;
		boolean t1 = true, t2 = true, t3 = true;
		for (int i = 2; i <= n; i++) {
			if (a[i] < a[i - 1]) {
				ind = i;
			}
		}
		if (ind == -1) {
			System.out.println(0);
			return;
		}
		for (int i = 2; i < ind; i++) {
			if (a[i - 1] > a[i]) {
				t1 = false;
				break;
			}
		}
		for (int i = ind + 1; i <= n; i++) {
			if (a[i - 1] > a[i]) {
				t2 = false;
				break;
			}
		}
		pw.println((t1 && t2 && a[1] >= a[n]) ? n - ind + 1 : -1);
		pw.close();
	}

	private static int nextInt() throws IOException {
		return Integer.parseInt(next());
	}

	private static long nextLong() throws IOException {
		return Long.parseLong(next());
	}

	private static double nextDouble() throws IOException {
		return Double.parseDouble(next());
	}

	private static String next() throws IOException {
		while (st == null || !st.hasMoreTokens())
			st = new StringTokenizer(br.readLine());
		return st.nextToken();
	}
}
