import java.util.Scanner;
public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int con = 1;
		int ans = 0;

		for (int i = 0; i < N; i++) {
			if (sc.nextInt() == con) {
				con++;
			}else {
				ans++;
			}
		}
		if (con == 1) {
			System.out.println(-1);
		}else {
			System.out.println(ans);
		}
	}
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz