/* package codechef; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
public class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
		Scanner input = new Scanner(System.in);
		int days = input.nextInt();
		int assignment = input.nextInt();
		int daysToComplete= 0;
		for(int i = 0;i<assignment;i++){
		    daysToComplete+=input.nextInt();
		}
		if(assignment>days || daysToComplete>days){
		    System.out.println("-1");
		}else {
		    System.out.println(days- daysToComplete);
		}
	}
}

/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