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