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.*;
import java.util.*;

public class Main {

  public static void main(String[] args) {
    InputReader sc = new InputReader(System.in);
    PrintWriter pw = new PrintWriter(System.out);
    Random gen = new Random();
    int test = 1;//sc.nextInt();
    while(test-- > 0)
    {
     	int n=sc.nextInt();
	int t=0,b=0;
	int a[]=new int[n];
	for(int i=0;i<n;i++)
	{
		a[i]=sc.nextInt();
	if(i>0&&a[i-1]>a[i])
	{
		t++;
		b=i;
	}
}
	if(t==0)
		pw.println("0");
	else if(t==1&&a[n-1]<=a[0])
		pw.println(n-b);
	else
		pw.println("-1");
        
        
        
    }
      
    pw.close();
  }






























































































































































































  static class InputReader {

    private final InputStream stream;
    private final byte[] buf = new byte[8192];
    private int curChar, snumChars;

    public InputReader(InputStream st) {
      this.stream = st;
    }

    public int read() {
      if (snumChars == -1)
        throw new InputMismatchException();
      if (curChar >= snumChars) {
        curChar = 0;
        try {
          snumChars = stream.read(buf);
        } catch (IOException e) {
          throw new InputMismatchException();
        }
        if (snumChars <= 0)
          return -1;
      }
      return buf[curChar++];
    }

    public int nextInt() {
      int c = read();
      while (isSpaceChar(c)) {
        c = read();
      }
      int sgn = 1;
      if (c == '-') {
        sgn = -1;
        c = read();
      }
      int res = 0;
      do {
        res *= 10;
        res += c - '0';
        c = read();
      } while (!isSpaceChar(c));
      return res * sgn;
    }

    public long nextLong() {
      int c = read();
      while (isSpaceChar(c)) {
        c = read();
      }
      int sgn = 1;
      if (c == '-') {
        sgn = -1;
        c = read();
      }
      long res = 0;
      do {
        res *= 10;
        res += c - '0';
        c = read();
      } while (!isSpaceChar(c));
      return res * sgn;
    }

    public int[] nextIntArray(int n) {
      int a[] = new int[n];
      for (int i = 0; i < n; i++) {
        a[i] = nextInt();
      }
      return a;
    }
    
    public static int[] shuffle(int[] a, Random gen)
    { for(int i = 0, n = a.length;i < n;i++)
        { int ind = gen.nextInt(n-i)+i;
        int d = a[i];
        a[i] = a[ind];
        a[ind] = d;
            
        }
        return a;
    }
    

    public String readString() {
      int c = read();
      while (isSpaceChar(c)) {
        c = read();
      }
      StringBuilder res = new StringBuilder();
      do {
        res.appendCodePoint(c);
        c = read();
      } while (!isSpaceChar(c));
      return res.toString();
    }

    public String nextLine() {
      int c = read();
      while (isSpaceChar(c))
        c = read();
      StringBuilder res = new StringBuilder();
      do {
        res.appendCodePoint(c);
        c = read();
      } while (!isEndOfLine(c));
      return res.toString();
    }

    public boolean isSpaceChar(int c) {
      return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }

    private boolean isEndOfLine(int c) {
      return c == '\n' || c == '\r' || c == -1;
    }

  }

}