function calculate() {
	var h = document.getElementById("h").value;
	var b = document.getElementById("b").value;
	var area = "";
	if (h != "" && b != "") {
		h = parseFloat(h);
		b = parseFloat(b);
		area = ((h * b)/2);
		document.getElementById("area").innerHTML = "area: " + area;
	}
}