import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {

  public static void main(String[] args) throws Exception {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    String[] head = br.readLine().split(" ");
    int n = Integer.parseInt(head[0]);
    int m = Integer.parseInt(head[1]);
    int sum = 0;
    String[] line = br.readLine().split(" ");
    for (int i = 0; i < m; i++) {
      sum += Integer.parseInt(line[i]);
    }
    System.out.println(Math.max(n - sum, -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