import java.util.Arrays;
import java.util.Comparator;
import java.util.Scanner;


public class Main {
	

	
	
	public static void main(String[] args) {

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