Angular Reactive Form Validation: Do not allow a number less than existing number

I have a numeric input field in Angular reactive form.

 <input id="referenceNumber" type="number" formControlName="referenceNumber" />
 <div *ngIf="form.referenceNumber.invalid && (form.referenceNumber.dirty || form.referenceNumber.touched)">
    <span *ngIf="form.referenceNumber.errors?.required">
       Please Enter a number.
    </span>
 </div>

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.

You set initial value on your input and you are use the built in Validators.min validator. You set as min number your initial value.

var initialValue: number;

this.form = this.fb.group({
    referenceNumber: [initialValue. 
    Validators.min(initialValue),
    Validators.pattern(/^[1-9]\d*$/) ])
});

html

<span *ngIf="myForm.get('referenceNumber').errors.min">
     numberInputName cannot be more than {{ initialValue }}. 
</span>

<span *ngIf="myForm.get('referenceNumber').errors?.pattern">
 numberInputName must be greater than zero
</span>