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.InputStreamReader;
import java.io.BufferedReader;
import java.io.IOException; 
import java.lang.reflect.Array;
import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;
 
 
 
    public class Codeforces {
        
        
 
        public static void main(String[] args) throws IOException {
           BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
           int n = Integer.parseInt(br.readLine());
          int[] arr = new int[n];
          String[] s = br.readLine().trim().split(" ");
          for(int i = 0; i < n; i++){
              arr[i] = Integer.parseInt(s[i]);
          }
          int max = arr[0];
          int index = 0;
          boolean NotPossible = false;
          for(int i = 0; i < n; i++){
              if(arr[i] < max){
                  break;
              }
              else {
                  max = arr[i];
                  index = i;
              }
          }
         // System.out.println(index);
          for(int i = index + 1; i < n  ; i++){
              if(i + 1 >= n && arr[i] > arr[0]){
                  NotPossible = true;
                  break;
              }   
              else if(i + 1 < n && arr[i] > arr[i + 1]){
                     NotPossible = true;
                     break;
                 }
          }
          
          if(NotPossible) System.out.println(-1);
          else if(index == n - 1) System.out.println(0);
          else System.out.print(n - 1 - index);
          
        }    
    }