function calculate(){
    var principal = document.loanform.principal.value;
    var interest = document.loanform.interest.value / 100 / 12;
    var payments = document.loanform.years.value * 12;
    var tax = document.loanform.tax.value;
    var x = Math.pow(1 + interest, payments);
    var monthly = (principal * x * interest) / (x - 1);
    
    if (!isNaN(monthly) &&
    (monthly != Number.POSITIVE_INFINITY) &&
    (monthly != Number.NEGATIVE_INFINITY)) {
        document.loanform.payment.value = round(monthly);
        document.loanform.total.value = round(monthly * payments);
        document.loanform.totalinterest.value = round((monthly * payments) - principal);
        document.loanform.mttl.value = round((tax / 12) + monthly)
    }
    
    else {
        document.loanform.payment.value = "";
        document.loanform.mttl.value = "";
        document.loanform.total.value = "";
        document.loanform.totalinterest.value = "";
    }
}

function round(x){
    return Math.round(x * 100) / 100;
}

function SubmitDocument(){
    if (loanform.principal.value == "") {
        alert("You must enter a Principal.");
        loanform.principal.focus();
        return false
    }
    else 
        if (loanform.interest.value == "") {
            alert("You must enter an Interest Rate.");
            loanform.interest.focus();
            return false
        }
        else 
            if (loanform.years.value == "") {
                alert("You must enter a Payment Period.");
                loanform.years.focus();
                return false
            }
            else 
                if (loanform.stry.value == "") {
                    alert("You must enter a Starting Year.");
                    loanform.stry.focus();
                    return false
                }
                else 
                    return true
}