import java.util.*;
import java.math.BigDecimal;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int n = sc.nextInt();
      
        int count = 1,tmp;
        int brick = 0;
        for(int i = 0;i<n;i++){
          tmp = sc.nextInt();
          if(tmp == count){
            count += 1;
          }else{
            brick += 1;
          }
        }
        if(count == 1){brick = -1;}
        System.out.println(brick);
        

        
    }
}
/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