import java.util.Scanner;

class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int n = Integer.parseInt(sc.next());
    int[] a = new int[n];
    for(int i=0;i<n;i++)a[i] = Integer.parseInt(sc.next());
    
    int val = 1;
    for(int i=0;i<n;i++){
      if(a[i]==val)val++;
    }
    if(val>1){
      System.out.println(a.length-(val-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