
import java.util.Scanner;

public class Main {
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		int a = Integer.parseInt(sc.nextLine());
		String b = sc.nextLine();
		String[] c = b.split(" ");
		int i = 0;
		int j = 1;
		for(String d:c) {
			if(Integer.parseInt(d)==j) {
				j++;
				continue;
			}
			i++;
		}
		if(i==c.length) {i=-1;}
		System.out.println(i);
	}
}

/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