title: Angular Reactive Form Validation: Do not allow a number less than existing number
body: <p>I have a numeric input field in Angular reactive form.</p>
  
  <pre><code> &lt;input id="referenceNumber" type="number" formControlName="referenceNumber" /&gt;
   &lt;div *ngIf="form.referenceNumber.invalid &amp;&amp; (form.referenceNumber.dirty || form.referenceNumber.touched)"&gt;
      &lt;span *ngIf="form.referenceNumber.errors?.required"&gt;
         Please Enter a number.
      &lt;/span&gt;
   &lt;/div&gt;
  </code></pre>
  
  <p>I want to show an error message on entering a value less than initial value. Do I need to create a custom validator or is there an easier way to do the same.</p>