import java.util.ArrayList;
import java.util.Scanner;

public class Main {
    public static void main(String... args) {
        Scanner scanner = new Scanner(System.in);
        int N = scanner.nextInt();
        ArrayList<Integer> list = new ArrayList<>();

        int cnt = 1;
        for (int i = 0; i<N; i++) {
            int sc = scanner.nextInt();
            if (sc == cnt) {
                list.add(cnt++);
            }
        }

        int rst =  N - list.size();
        System.out.println(rst == N ? -1 : rst);
    }
}
/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