import java.util.Scanner;

public class  Main {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);

		int n = scanner.nextInt();
		int m = scanner.nextInt();

		for(int i = 0; i < m; i++) {
			n -= scanner.nextInt();
		}

		if(n < 0) {
		System.out.print(-1);
		} else {
			System.out.print(n);
		}
	}
}
/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