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.OutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 * @author Anirudh "code_overlord" Rayabharam
 */
public class Main {
	public static void main(String[] args) {
		InputStream inputStream = System.in;
		OutputStream outputStream = System.out;
		InputReader in = new InputReader(inputStream);
		PrintWriter out = new PrintWriter(outputStream);
		TaskB solver = new TaskB();
		solver.solve(1, in, out);
		out.close();
	}
}

class TaskB {
    public void solve(int testNumber, InputReader in, PrintWriter out) {
        int n = in.nextInt();
        int[] a = new int[n];
        for (int i = 0; i < n; i++) {
            a[i] = in.nextInt();
        }

        int mis = 0, misIdx = -1;
        for (int i = 1; i < n; i++) {
            if (a[i] < a[i-1]) {
                ++mis;
                if (mis > 1) {
                    out.println(-1);
                    return;
                }

                misIdx = i;
            }
        }

        if (mis == 0) out.println(0);
        else if (a[n-1] <= a[0]) {
            out.println(n - misIdx);
            return;
        } else out.println(-1);
    }
}

class InputReader {
    public BufferedReader reader;
    private int tokenCount, nextTokenIndex;
    private String[] tokens;

    public InputReader(InputStream stream) {
        reader = new BufferedReader(new InputStreamReader(stream));
        tokenCount = nextTokenIndex = 0;
    }

    public String next() {
        String nextLine;
        if (nextTokenIndex == tokenCount) {
            try {
                nextLine = reader.readLine();
                nextTokenIndex = 0;
                tokens = nextLine.split("\\s");
                tokenCount = tokens.length;
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }

        return tokens[nextTokenIndex++];
    }

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

}

