import java.util.Scanner;

public class Main{
  public static void main(String[] args){
    
    Scanner sc = new Scanner(System.in);
    
    int N = sc.nextInt();
    int M = sc.nextInt();
    
    int sumA = 0;
    
    for(int i=0; i<M; i++){
      sumA += sc.nextInt();
    }
    
    if(sumA > N){
      System.out.print("-1");
    }else if(sumA == N){
      System.out.print("0");
    }else{
      System.out.print(N - sumA);
    }
    
  }
}
/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