import java.util.Scanner;

class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();
    int m = scan.nextInt();
    int a[] = new int[m];
    int sum = 0;
    for(int i = 0; i<m; i++){
      a[i] = scan.nextInt();
    }
    for(int i = 0; i<m; i++){
      sum += a[i];
    }
    if(sum <= n){
      System.out.println(n - sum);
    }else{
      System.out.println(-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