import java.util.Scanner;
public class Main{
	public static void main(String[] args){
		Scanner input = new Scanner(System.in);
		int num = input.nextInt();
		int n  = input.nextInt();
		int[] arr = new int[n];
		for (int i = 0; i < arr.length; i++){
			arr[i] = input.nextInt();
		}
		int sum = 0;
		for(int i = 0; i < arr.length;i++){
			sum +=arr[i];
		}
		if (num - sum >= 0)
			System.out.println(num - sum);
		else
			System.out.println("-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