<script type="text/javascript">
<!--
function prepare_form(obj){
    var arr_elements = obj.form.elements;
    var varr=obj.form.variant.value;
    if ((varr >= 1) && (varr <= 30)){
        obj.form.action = varr+".html";
    }
    else{
        obj.form.action = "404.html";
    }
    return true;
}
// -->
</script>
    <form name="form1" action="/" method="post">
        Номер варианта (1-30):
        <input type="text" name="variant"  size="2" maxlength="2">
        <input type="submit" value="Найти" onclick="prepare_form(this)">
    </form>