Allowing Only Certain Numbers and/or Increments of Numbers In Form Field

[Ntrimgs]

I have a form field which only allows increments of 25 to be entered into the form field, but now I need it to also allow the number 0. Here is the existing code:

<input type="text" size="3" onChange="this.value = Math.max(Math.round(parseInt(0 + this.value, 10) / 25) * 25, 50);" name="product_qty" value="50" />';

Anyone know how to make this allow 0??