import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int holidays = sc.nextInt();
        int homeworks = sc.nextInt();

        int sum = 0;
        for (int i=0; i<homeworks; i++) {
            sum += sc.nextInt();
        }

        if (sum > holidays) System.out.println(-1);
        else System.out.println(holidays - sum);
    }
}

/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