import java.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String line = sc.nextLine();
      	String num = sc.nextLine();
		String[] data = line.split(" ");
      	String[] num2 = num.split(" ");
      	Integer sum = 0;
      	for(int i = 0; i < Integer.parseInt(data[1]); i++){
          sum += Integer.parseInt(num2[i]);
        }
		Integer nokori = Integer.parseInt(data[0]) - sum;
      if(nokori < 0){
        System.out.println(-1);
      }else{
        System.out.println(nokori);
      }
	}
}
/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