import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long n;
		int m = 0;
		long ans = 0;
		long sum = 0;

		n = sc.nextLong();
		m = sc.nextInt();

		for (int i = 0; i < m; i++) 
			sum += sc.nextInt();
		

		if (n - sum >= 0)
			ans = n - sum;
		else
			ans = -1;
		System.out.print(ans);
		sc.close();
	}
}
/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