How to add an if statement to check multiple variables in a function (Javascript)

I am very new to JavaScript and am learning how to work with functions and if statements. There are many similarly-named questions but none relate to this that I see. I have this block of code in my HTML doc that serves as a function to perform a calculation. It prompts the user to enter the number of rooms and total sq. ft., then multiplies the sq ft by 5 to get the estimate. I am trying to figure out how to add an if-statement to serve as an error message if a user inputs a negative number. Below is the function copied from the code. Would I add it directly in the function, before or after it? What I basically want to say is,

if (var a and b <= 0) {
print "Invalid entry. Please reload the page and enter a positive number." } 

How would I go about doing this in JavaScript? Also, is there a better way to ask the user than using < p> < /p>?

<div class="container-fluid">
    <p>Please enter the number of rooms affected, and the total square foot. </p>
    <input type="text" name="numRooms" id="numRooms"> <br>
        
    <input type="text" name="roomSize" id="roomSize"><br>
    <button type="button" onclick="submit1()">submit</button><br>

    <p id="result"></p>
</div>

<script>
    function submit1(){
        var a =  document.getElementById("numRooms").value;
        var b =  document.getElementById("roomSize").value;
        var c =  parseInt(b) * 5; 

        document.getElementById("result").innerHTML="Your estimate is : $" + c;
    }
    </script>