import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        int IMPOSSIBLE = -1;
        boolean isPossible = false;
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int unbreakNum = 1;
        int unbreakCount = 0;

        for (int i = 0; i < n; i++) {
            if (sc.nextInt() == unbreakNum) {
                isPossible = true;
                unbreakCount++;
                unbreakNum++;
            }
        }

        if (!isPossible) {
            System.out.println(IMPOSSIBLE);
        } else {
            System.out.println(n - unbreakCount);
        }
    }
}
/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