import java.util.*;
import java.lang.*;
import java.io.*;

class Main
{
    public static void main (String[] args) throws java.lang.Exception {
        Scanner io = new Scanner(System.in);
        int n = io.nextInt();
        int arr[] = new int[n];
        for(int i = 0; i < n; i++)
            arr[i] = io.nextInt();
        int f = 1;
        for(int i = 0; i < n; i++)
            if(arr[i] == f && f++ > 0);
        System.out.println(f == 1 ? -1 : n-f+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