Variable Doesn't Work In An Object Statement

I have an input form that accepts numbers. I am trying to compare the last field value to the current field value. My problem is that when I use the variable name of the last field in an object path, I get the error "null or not an object". The var 'L' is the problem in the statement below. If I manually type the field name in place of 'L', it works fine. How do I have to write this statement so 'L' works?

Problem statement:

c=window.document.save_info.L.value;

Here is my code: Form element:

Code:

<input style="border:1px solid gray; background-color: #FF9;" onkeyup="moveOnTwo('frm_score1',<?PHP echo $num_holes; ?>,this,'frm_score2')" size="1" maxlength="3" name="frm_adjsc1" id="frm_adjsc1" />

JavaScript Function:

Code:

function moveOnTwo(lastFieldID,holes,field,nextFieldID){
var L = lastFieldID ; var N = nextFieldID;

[code]...