import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        long num, x, sum=0;
        Scanner scanner = new Scanner(System.in);
        num = scanner.nextLong();
        x = scanner.nextLong();
        //scanner.nextLine();
        for(int i=0; i<x; i++) sum += scanner.nextLong();
        if(sum > num){
            System.out.println(-1);
        }
        else System.out.println(num - 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