import java.util.*;
public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int a[] = new int[N];
    int answer = 0;
    int tmpNumber=1;
    boolean isMinusOne = true;
    for(int i=0;i<N;i++){
      a[i]=sc.nextInt();
      if(a[i]==1){
        isMinusOne=false;
      }
    }
    if(isMinusOne){
      System.out.println(-1);
      return;
    }
    
    
    for(int i=0;i<N;i++){
      if(a[i]!=tmpNumber){
        answer=answer+1;
      }else{
        tmpNumber=tmpNumber+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