import java.io.*;
import java.util.*;
public class Main {
	public static void main (String[] args) {
	    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 c=1;
	boolean f=false;
	for(int i=0;i<n;i++){
	    if(a[i]==c)
	    {
	        c++;
	        f=true;
	    }
	}
	if(f)
	System.out.println(n-c+1);
	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