import java.util.*;
import java.io.*;
import java.lang.*;
public class Main{

public static void main(String[] args) throws FileNotFoundException, IOException{
       Scanner s=new Scanner(System.in);
       BufferedWriter out=new BufferedWriter(new OutputStreamWriter(System.out));
        int n=s.nextInt();
      // Stack<Integer>stk=new Stack<>();
       int res=-1,j=1;
       for(int i=0;i<n;i++){
       	int x=s.nextInt();
       	if(x==j){j++;}
       }
       if(j!=1) res=n-(j-1);
        out.write(res+"\n");
        out.flush();
  }
}

/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