function MakeNum(num,i,s) 
{     
		s = Math.floor(num)%10+(s+'');
        if(num>10) 
		{
                if(i>0 && (i%3==0))s=","+s;
                s=MakeNum(num/10,i+1,s);
        }
        return s;
}

function MakeMoney(num) 
{
        n=Math.round(num*100);
        d2=Math.floor(n%10);
        d1=Math.floor((n%100)/10);
        return "$ "+MakeNum(num,1,".")+d1+d2
}

function compute(form) 
{
        var a = parseFloat(form.tfAmount.options[form.tfAmount.selectedIndex].value);
        var r = parseFloat(form.tfRate.options[form.tfRate.selectedIndex].value);
        var l = parseFloat(form.tfLen.options[form.tfLen.selectedIndex].value);
        var n = parseFloat(form.tfInflation.options[form.tfInflation.selectedIndex].value);
        r = r - n;
        var k = 1/(1+r);
        var m = 1;
        for(var i=0;i<l;i++) m = m * k;
        form.tfPay.value = MakeMoney(a*(1-m)/r);
}