import java.util.*;
import java.lang.*;
import java.io.*;

public final class Main
{
    public static void main (String[] args) throws java.lang.Exception {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
      int m = in.nextInt();
       int sum = 0;
      for(int i =0; i< m; i++)
         sum += in.nextInt();
      int re = n- sum;
      if(re<0){
        System.out.println(-1);
      }else {
       System.out.println(re);
      }
    }
}
/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