import java.util.Scanner;
public class Main{
	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		int n = scan.nextInt();
		int m = scan.nextInt();
		int[] array = new int[m];
		for(int i = 0; i < m; i++){
			array[i] = scan.nextInt();
		}
		int asobi = n;
		for(int i = 0; i < m; i++){
			asobi -= array[i];
		}
		if(asobi < 0){
			asobi = -1;
		}
		System.out.println(asobi);
	}
}
/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