import java.util.Scanner;

public class Main {

    public static void main(String[] args)  {
        //input
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();

        int broken = 0;
        int idx = 1;
        for (int i = 0; i < n; i++) {
            int num = sc.nextInt();
            if (num != idx) {
                broken++;
            } else {
                idx++;
            }
        }
        if ((n - broken) == idx - 1 && idx > 1) {
            System.out.println(broken);
        } else {
            System.out.println("-1");
        }
    }
}
/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