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