import java.util.*;

class Main{
  public static void main(String[] args){
    Scanner sc=new Scanner(System.in);
    int N=sc.nextInt();
    int count=1;
    int Answer=0;
    
    for(int i=0;i<N;i++){
      if(sc.nextInt()==count){
        count++;
      }else{
        Answer++;
      }
    }
    
      if(Answer==N){
        Answer=-1;
      }
    
    System.out.println(Answer);
    
  }
}
/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