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.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;

public class Codeforces454B {
	public static void main(String[] args) throws IOException {
		InputReader ir = new InputReader(System.in);
		PrintWriter out = new PrintWriter(System.out);

		int n = ir.nextInt();
		
		int[] array = new int[2 * n + 5];
		
		for (int i = 0; i < n; i++) {
			array[i + n] = ir.nextInt();
		}
		
		int head = n;
		int tail = 2 * n - 1;
		
		int shifts = n - isOrdered(array, head, tail); 
		
		if (shifts != 0) {
			for (int i = 0; i < shifts; i++) {
				int temp = array[tail];
				array[tail] = 0;
				array[head - 1] = temp;
				head--;
				tail--;
			}
		}
		
		if (isOrdered(array, head, tail) == n) {
			out.println(shifts);
		} else {
			out.println(-1);
		}
		
		out.flush();
	}
	
	private static int isOrdered(int[] list, int head, int tail) {
		int count = 1;
		for (int i = head + 1; i <= tail; i++) {
			if (list[i] < list[i - 1]) {
				break;
			}
			count++;
		}
		return count;
	}
	
    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

    }
}
