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();

        int[] A = new int[M];
        for(int i = 0; i < A.length; i++) {
            A[i] = scanner.nextInt();
            N -= A[i];
        }

        if(N >= 0) {
            System.out.println(N);
        } else {
            System.out.println(-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