import java.util.Scanner;

public class Main {
    public static void main(String[] args){

        Scanner sc = new Scanner(System.in);

        int a = sc.nextInt();
        int[] b = new int[a];
        int answer = 0;

        for(int i=0;i<a;i++){
            if(sc.nextInt() == answer + 1)
                answer++;
        }
        if(answer == 0)
            System.out.println(-1);
        else
            System.out.println(a-answer);
    }
}
/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