import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Neighbour {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        int size = Integer.parseInt(scanner.nextLine());
        String[] input = scanner.nextLine().split(" ");
        List<Integer> result = new ArrayList<>();
        for(int i = 0; i < size;i++){
            result.add(Integer.parseInt(input[i]));
        }
          int index = 0;
        for(int i = 0; i < result.size() -1 ;i++){
            if(result.get(i) > result.get(i + 1)){
                index = -1;
            }else{
                 index = result.indexOf(result.get(i + 1));
                break;
            }
        }
        System.out.println(index);
    }
}