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

   for (int i = 0;i<N;i++) ai[i] = sc.nextInt();
   int ans = 0;
   
   int brick = 1; 
   for(int i = 0;i<N;i++)
   {
     if(ai[i]==brick) brick++;
     else ans++;
   }
   if(ans==N&&brick==1) ans = -1;
   System.out.println(ans);
 }
}
/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