Algo muy simple que no se hacer!!!

[bet]

Algo así te puede servir?

<pre>
&lt;html&gt;
&lt;head&gt;
&lt;title&gt; New Document &lt;/title&gt;

&lt;script language=&quot;JavaScript&quot;&gt;
&lt;!--
var contador = 3;
function suma(){
document.forms[0].cmpo1.value = eval(document.forms[0].cmpo1.value) + contador;
}
//--&gt;
&lt;/script&gt;&lt;/head&gt;

&lt;body bgcolor=&quot;#FFFFFF&quot;&gt;
&lt;form method=post action=&quot;&quot;&gt;
&lt;input type=&quot;text&quot; name=&quot;cmpo1&quot; value=0&gt;
&lt;input type=&quot;button&quot; onclick=&quot;suma();&quot;&gt;

&lt;/form&gt;
&lt;/body&gt;
&lt;/html&gt;
[/cont] </pre>

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