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.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.math.BigDecimal;
import java.io.BufferedWriter;
import java.io.BufferedReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.Writer;
import java.util.StringTokenizer;
import java.math.BigInteger;
import java.io.InputStream;

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

class TaskB {
    public void solve(int testNumber, FastScanner in, FastWriter out) {
        int n = in.nextInt();
        int[] ns = in.nextIntArray(n);

        int n1, n2;

        int nseqs = 1;
        int sizeLast = 1;

        int first = ns[0];
        for (int i = 1; i < n; i++) {
            if (ns[i] < first) {
                nseqs++;
                sizeLast = 1;
            } else {
                sizeLast++;
            }
            first = ns[i];
        }

        if (nseqs == 2 && ns[0] >= ns[n - 1]) {
            out.println(sizeLast);
        } else if (nseqs == 1) {
            out.print(0);
        } else {
            out.println(-1);
        }
    }
}

class FastScanner {
    private BufferedReader reader;
    private StringTokenizer tokenizer;

    public FastScanner(InputStream stream) {
        this.reader = new BufferedReader(new InputStreamReader(stream));
        this.tokenizer = new StringTokenizer("");
    }

    public String next() {
        return  ensureTokens() ? tokenizer.nextToken() : null;
    }

    private boolean ensureTokens() {
        while(!tokenizer.hasMoreTokens()){
            try{
                tokenizer = new StringTokenizer(reader.readLine());
            } catch (Exception e){
                return false;
            }
        }
        return true;
    }

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

    public int[] nextIntArray(int size) {
        int[] array = new int[size];
        for (int i = 0; i < size; i++) {
            array[i] = nextInt();
        }
        return array;
    }

}

class FastWriter {
    private final PrintWriter printWriter;

    public FastWriter(OutputStream stream) {
        printWriter = new PrintWriter(new BufferedWriter(new OutputStreamWriter(stream)));
    }

    public FastWriter(Writer writer) {
        printWriter = new PrintWriter(writer);
    }

    public void close() {
        printWriter.close();
    }

    public void print(int element) {
        printWriter.print(element);
    }

    public void println(int element) {
        printWriter.println(element);
    }

}

