import java.util.*;


public class Main {
    public static void main(String[] args) {
      
      Scanner sc = new Scanner(System.in);
      int n = Integer.parseInt(sc.next());
      int m = Integer.parseInt(sc.next());
      int sum = 0;
      int[] ary = new int[m];
      for (int i = 0; i < ary.length; i++) {
        ary[i] = Integer.parseInt(sc.next());
        sum += ary[i];
      }
      
      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