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<script type="text/javascript">
    function updateSum(){
        var fields = document.getElementsByTagName('input');
        var sum = 0;
        for(var i=0;i<=fields.length;i++){
            if(fields[i].id.substring(0,5)=='field'){
                sum = parseInt(fields[i]) + parseInt(sum);
            }
        }
        document.getElementById('sum').value = sum;
    }
</script>

<input type="text" name="field1" id="field1" onkeyup="updateSum()"><br>
<input type="text" name="field2" id="field2" onkeyup="updateSum()"><br>
<input type="text" name="field3" id="field3" onkeyup="updateSum()"><br>
<input type="text" id="sum" name="sum">

function updateSum(){
   var fields = document.getElementsByTagName('input');
   var sum = 0;
                
   for(var i=0;i<fields.length;i++){
      if(fields[i].id.substring(0,5)=='field' && parseInt(fields[i].value)>0){
         sum = parseInt(fields[i].value) + parseInt(sum);
      }
   }
   document.getElementById('sum').value = sum;
}