import java.util.Scanner;

public class Main {

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

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

        int search = 1;
        int broken = 0;
        for (int i = 0; i < n; i++) {
            if (array[i] == search) {
                search++;
                continue;
            }
            broken++;
        }

        if (broken == n) broken = -1;
        System.out.println(broken);
    }

}
/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