import java.util.Scanner;

public class Main {

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

		int n = sc.nextInt();
		int[] nums = new int[n];
		int[] pos  = new int[n + 1];

		for (int i = 0; i < n; i++) {
			nums[i] = sc.nextInt();
			pos[nums[i]] = i;
		}

		//System.out.println(Arrays.toString(nums));
		//System.out.println(Arrays.toString(pos));

		System.out.println(solve(nums, pos));

		sc.close();
	}

	public static int solve (int[] nums, int[] pos) {
		int lim = nums.length;
		for (int i = 0; i < lim; i++) {
			if (isSorted(nums)) {
				return i;
			} else {
				int p = pos[i + 1];
				nums[p] = -1;
			}
		}
		return -1;
	}

	public static boolean isSorted (int[] nums) {
		int head = -1;
		for (int i = 0; i < nums.length; i++) {
			if (nums[i] == -1) continue;
			if (head == -1) {
				head = nums[i];
				continue;
			} else if (head > nums[i]) {
				return false;
			} else {
				head = nums[i];
			}
		}
		return true;
	}

}