import java.util.*;
 
public class Main{
  public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N =sc.nextInt();
		int M = sc.nextInt();
		int total = 0;
		for(int i = 0;i < M;i++) {
			int x = sc.nextInt();
			total += x;
		}
		if(N - total < -1) {
			System.out.println("-1");
		}else {
			System.out.println(N - total);
		}
	}
}
/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