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 A[]=new int[M];
		for(int i=0;i<M;i++) {
			A[i]=scan.nextInt();
		}
		int day=0;
		for(int i=0;i<M;i++) {
			day+=A[i];
		}
		if(N-day<0) {
			System.out.println("-1");
		}
		else {
			System.out.println(N-day);
		}

	}

}
/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