import java.util.Scanner;
public class Main{
	static Scanner scan = new Scanner(System.in);
	public static void main(String[] args) {
		System.out.println(answer());
	}
	public static int answer(){
		int n = scan.nextInt();
		int m = scan.nextInt();
		int[] arr= new int[m];
		int sum = 0;
		for(int i = 0 ; i < m ; i++){
			arr[i] = scan.nextInt();
			sum += arr[i];
		}
		return n >= sum ?n-sum:-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