import java.util.*;
import java.io.*;
public class Main
{
  public static void main(String args[])throws Exception
  {
    BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
    PrintWriter pw=new PrintWriter(System.out);
    String str[]=br.readLine().split(" ");
    int n=Integer.parseInt(str[0]);
    int m=Integer.parseInt(str[1]);
    int hw=0;
    str=br.readLine().split(" ");
    for(int i=0;i<m;i++)
    hw+=Integer.parseInt(str[i]);
    if(hw>n)
    pw.println(-1);
    else
    pw.println(n-hw);
    pw.flush();
    pw.close();
  }
}
/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