import java.util.Scanner;
import java.lang.Math;
class Main {
    public static void main(String args[]) {
      Scanner scan = new Scanner(System.in);
      int n = scan.nextInt();
      int m = scan.nextInt();
      int sum = 0;
      for(int i = 0; i < m; i++){
          sum += scan.nextInt();
          
      }
      if(n-sum < 0){
          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