function update(str) {
   document.getElementById('button').visibility = "hidden";
   document.getElementById('result').innerHTML = str;
}

function calculate() {
    var height = (parseInt(document.getElementById("heightf").value, 10) * 12) + parseInt(document.getElementById("heighti").value, 10);
    var weight = parseInt(document.getElementById("weight").value, 10);
    if (isNaN(height) || isNaN(weight)) {
        update("Error parsing input.");
    } else if (height === 0) {
        update("Height can not be zero.");
    } else {
        var bmi = (weight / (height * height)) * 703;
        var cat;
        if (bmi < 18.5) { cat = "underweight.";}
        else if (bmi < 24.9) { cat = "normal.";}
        else if (bmi < 29.9) { cat = "overweight.";}
        else { cat = "obese.";}
        update("BMI: " + bmi.toFixed(1).toString() + "<br />You are " + cat);
    }
}