import java.util.*;


public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] brick = new int[N];

        for(int i = 0; i < N; i++)
          brick[i] = Integer.parseInt(sc.next());

        int count = 1;
        for(int i = 0; i < N; i++){
          if(brick[i] == count)
            count++;
        }

        if(count == 1)
          System.out.println(-1);
        else
          System.out.println(N-count+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