How can making watermark, math equation?

<label for="name">Please write your real estate's sqm size</label>
<input type="text" id="sqm" name="sqm" required
       minlength="4" maxlength="8" size="10" > 
<label for="name">Please price of real estate</label>
<input type="text" id="price" name="price" required
       minlength="4" maxlength="8" size="10" >

Basically what I want to achieve is to take the input from the SQM size from the first input and take the price of the real estate values. After taking both values dividing them in this order Price/SQM, I want to get the price per sqm of the real estate automatically, so I can connect it to my backend.