import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int n = sc.nextInt();
		int m = sc.nextInt();
		int sum = 0;
		for (int i = 0; i < m; i++) {
			int a = sc.nextInt();
			sum = sum + a;
		}
		
		if (n < sum) {
			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