
import java.util.Scanner;

public class Main {
	public static void main (String[] args) {
		int x;
		int y;
		int z;
		Scanner scan = new Scanner(System.in);
		String num = scan.nextLine();
		String[] numbers = num.split(" ");
		x = Integer.parseInt(numbers[0]);
		y = Integer.parseInt(numbers[1]);
//		z = Integer.parseInt(numbers[2]);

		String A = scan.nextLine();
		String[] a = A.split(" ");

		for(int i=0; i<y; i++) {
			x = x-Integer.parseInt(a[i]);
		}

		if(x<0) {
			System.out.println(-1);
		}else {
			System.out.println(x);
		}

	}
}
/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