import java.util.*;

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