import java.util.*;

class Main{
 public static void main(String args[]){
   Scanner scn = new Scanner(System.in);
   int n = scn.nextInt();
   List<Integer> li = new ArrayList<>();
   for(int i = 0; i<n; i++){
	 li.add(scn.nextInt());
   }
   if(!li.contains(1)){
     System.out.println(-1);
     return;
   }
   int breaking = 0;
   int currentNo = 1;
   while(li.contains(currentNo)){
     breaking += li.indexOf(currentNo);
	 li = li.subList(li.indexOf(currentNo)+1, li.size());
     currentNo++;
   }
   breaking += li.size();
   System.out.println(breaking);
 }
}
/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