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

public class Main {
	public static void main (String[] args) {
		Scanner sc = new Scanner (System.in);
		int n = sc.nextInt(); int m = sc.nextInt(); int homeworkDays = 0;
		int[] arr = new int[m];
		while (m-- > 0) {
			arr[m] = sc.nextInt();
			homeworkDays+=arr[m];
		}
			int vacation = n-homeworkDays;
			System.out.println(vacation >= 0 ? vacation : "-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