WGN,
<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf8">
</head>
<body>
<input type="number" id="a" placeholder="Введите 'а' "><br/>
<input type="number" id="h" placeholder="Введите 'h' "><br/>
<input type="button" value="Считать" id="CALC" onclick="CalcFunc()"><br/>
<p id="result"></p>
<script>
function CalcFunc(){
var a = +document.getElementById("a").value||0;
var h = +document.getElementById("h").value||0;
a < 0 && (a = 0);
h < 0 && (h = 0)
var s=0.5*a*h
document.getElementById("result").innerHTML = "Результат: " + s;
}
</script>
</body>
</html>