function calculateRoomSize()
{	
	var width = document.getElementById("roomwidth").value;
	var length = document.getElementById("roomlength").value;
	var height = document.getElementById("roomheight").value;
	
	if ( parseInt(width) > 0 && parseInt(length) > 0 && parseInt(height) > 0 )
	{
		document.getElementById("roomsize").innerHTML = width * length * height;
	}
	else
	{
		alert("You must enter positive numbers for the room width, length and height.");
	}
}