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

/* Name of the class has to be "Main" only if the class is public. */
class Main
{

    public static void main (String[] args) throws java.lang.Exception
	{
		Scanner s=new Scanner(System.in);
		int n=s.nextInt();
		int a[]=new int[n];
		for(int i=0;i<n;i++)
		a[i]=s.nextInt();
		int count=0;
		boolean flag=false;
		int j=1;
		for(int i=0;i<n;i++)
		{if(a[i]==j)
		{j++;flag=true;}
		else
		count++;}
	if(flag)
		System.out.println(count);
		else
		{System.out.println("-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