import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ

		Scanner sc = new Scanner(System.in);
        
		int n = sc.nextInt();
		int m = sc.nextInt();
		int a[]=new int[m];
		
		for(int i=0;i<m;i++) {
		 a[i]=sc.nextInt();
		}
		int s=0;
		for(int i=0;i<m;i++) {
			s+=a[i];
		}
		if(s>n){
		System.out.println(-1);
		}else{
		System.out.println(n-s);
		}
	}
}
/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