import java.util.Scanner;
public class Main{
	public static void main(String[] args){
	  Scanner sc = new Scanner(System.in);
	  long n = sc.nextLong();
	  int m = sc.nextInt();
	  for(int i = 0; i < m; i++) {
		  n -= sc.nextLong();
		  if(n < 0) {
			  n = -1;
			  break;
		  }
	  }
	  System.out.println(n);
	}
}
/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