
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long N = sc.nextLong();
        int M = sc.nextInt();
        long[] A = new long[M];
        long ans = N;
        for(int i=0;i<M;i++){
            A[i] = sc.nextInt();
            ans -= A[i];
        }
        System.out.println(ans >= 0 ? ans : -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