import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        // write your code here
        int N;
        int M;
        int i = 0;
        int Sum = 0;


        Scanner scan = new Scanner(System.in);
        N = scan.nextInt();

        M = scan.nextInt();

        int[] A = new int[M];

        while (i < M) {
           
            A[i] = scan.nextInt();
            Sum = Sum + A[i];
            i = i + 1;
        }
        int Days = N - Sum;

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

        }
    }
}

/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