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

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
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 index = getIndex(a, 0);

        if (index >= a.length){
            out.println(0);
        } else {
            int secondIndex = getIndex(a,index);

            if (secondIndex == a.length && a[a.length-1] <= a[0]){
                out.println(a.length -  index);
            } else {
                out.println(-1);
            }
        }

    }

    private int getIndex(int[] a, int first) {
        int prev = a[first];
        int index = first+1;


        while (index < a.length){
            if (a[index] < prev){
                return index;
            }

            prev = a[index];
            index++;
        }

        return index;
    }
}

class InputReader {
    public BufferedReader reader;
    public StringTokenizer tokenizer;


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

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

        return tokenizer.nextToken();
    }

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

}

