import java.util.HashMap;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        HashMap<String,Integer> hash = new HashMap<>();
        int n = sc.nextInt();
        int m = sc.nextInt();
        long work = 0;
        for (int i=0;i<m;i++)work+=sc.nextInt();
        if (n<work)System.out.println(-1);
        else System.out.println(n-work);
    }
}

/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