Calculating Sum Of Text Fields - N Elements

They are not very expert in javascript. I created a script that needs to calculate the sum of text fields. My function is limited to 5 elements, you can simplify and make it work for n elements.

PHP Code:
<script>
function somma() {
var a=eval(document.getElementById("primo").value);
var b=eval(document.getElementById("secondo0").value);
var somma0=(a) - ((a) * (b)) / 100;
var somma0=Math.round(somma0 * 100) / 100;
document.getElementById("somma0").value=somma0;�
var b1=eval(document.getElementById("secondo1").value);
var somma1=(a) - ((a) * (b1)) / 100;
var somma1=Math.round(somma1 * 100) / 100;
document.getElementById("somma1").value=somma1;�
var b2=eval(document.getElementById("secondo2").value);
var somma2=(a) - ((a) * (b2)) / 100;
var somma2=Math.round(somma2 * 100) / 100;
document.getElementById("somma2").value=somma2;�
var b3=eval(document.getElementById("secondo3").value);
var somma3=(a) - ((a) * (b3)) / 100;
var somma3=Math.round(somma3 * 100) / 100;
document.getElementById("somma3").value=somma3;�
var b4=eval(document.getElementById("secondo4").value);
var somma4=(a) - ((a) * (b4)) / 100;
var somma4=Math.round(somma4 * 100) / 100;
document.getElementById("somma4").value=somma4;�
}
</script>
<input type="text" id="primo" onKeyUp="somma()" /><br /><br />
<?php for($i=0;$i<=4;$i++) { ?>
<input type="text" id="secondo<?php echo $i; ?>" name="secondo" onKeyUp="somma()" value="6" />
<input type="text" id="somma<?php echo $i; ?>" /><br />
<?php } ?>