Allow user to add negative number

I have this javascript that I need to modify so the first condition would allow me to input negative number -3, -2, -1..

    function testValidity(box, maxScale, useNA, useNegativeScale, nextBox)
    {
        var nScale, minScale, tab, oldValue, space, backspace, del, minNumPad, maxNumPad;
        oldValue = box.value;
        
        tab = 9;
        space = 32;
        backspace = 8;
        del = 46;
        
        if (useNegativeScale == "True")
        {
            var dash, subtract, hyphen;
            
            dash = 189;
            subtract = 109;
            hyphen = 173;
            nScale = 78;
            
            minScale = 48;
            minNumPad = 96;
            
            maxNumPad = minNumPad + (maxScale - minScale);
            
            if(event.keyCode == space || event.keyCode == backspace || event.keyCode == del)
            {
                event.cancelBubble = true;
                event.returnValue = false;
                box.value = "";
            }
            else if(event.keyCode == nScale && useNA == "True")
            {
                event.cancelBubble = true;
                event.returnValue = false;
                box.value = "n";
            }
            else if(event.keyCode == dash || event.keyCode == subtract || event.keyCode == hyphen)
            {
                event.cancelBubble = true;
                event.returnValue = false;
                box.value = "-";
            }
            else if((event.keyCode >= minScale && event.keyCode <= maxScale) || (event.keyCode >= minNumPad && event.keyCode <= maxNumPad))
            {
                if (event.keyCode > maxScale)
                {
                    box.value = event.keyCode - 96;
                }
                else
                {
                    box.value = event.keyCode - 48;
                }
                
                eval("document.all." + nextBox).focus();
                event.cancelBubble = true;
                event.returnValue = false;
            }
            else if (event.keyCode != tab)
            {
                event.cancelBubble = true;
                event.returnValue = false;
                box.value = oldValue;
                self.alert("<%= pageVals.Item("errors")("invalidvalue") %>");
                box.select();
            }
        }
        else
        {
            minScale = 49;
            minNumPad = 97;
            nScale = 78;
            
            maxNumPad = minNumPad + (maxScale - minScale);
            
            if(event.keyCode == space || event.keyCode == backspace || event.keyCode == del)
            {
                event.cancelBubble = true;
                event.returnValue = false;
                box.value = "";
            }
            else if(event.keyCode == nScale && useNA == "True")
            {
                event.cancelBubble = true;
                event.returnValue = false;
                box.value = "n";
            }
            else if((event.keyCode >= minScale && event.keyCode <= maxScale) || (event.keyCode >= minNumPad && event.keyCode <= maxNumPad))
            {
                if (event.keyCode > maxScale)
                {
                    box.value = event.keyCode - 96;
                }
                else
                {
                    box.value = event.keyCode - 48;
                }
                
                eval("document.all." + nextBox).focus();
                event.cancelBubble = true;
                event.returnValue = false;
            }
            else if(event.keyCode != tab)
            {
                event.cancelBubble = true;
                event.returnValue = false;
                box.value = oldValue;
                self.alert("<%= pageVals.Item("errors")("invalidvalue") %>");
                box.select();
            }
        }
    }

Code (markup):

This means if useNegativeScale, then this should allow me to input negative number from -3 to -1... But not sure how I can do that? Because everytime I input negative number it will remove.

Here is the HTML code:

<td style="text-align: center;"><input type="text" onFocus="getElement('TR<%= i %>').className = 'Highlite<%= TRClass %>';" onBlur="getElement('TR<%= i %>').className = '<%= TRType & TRClass %>';" onKeyDown="testValidity(this, '<%= asc(negativeScaleMax) %>', '<%= useNA %>', '<%= useNegativeScale %>', '<% if i = maxQuestion then response.write "sc_scores_" & i else response.write "sc_scores_" & (i + 1) end if %>');" name="sc_scores_<%= i %>" id="sc_scores_<%= i %>" maxlength="2" size="2" value="<%= negativeScaleNum %>" tabIndex="<%= i + 5 %>" /></td>

Code (markup):

Please help.

 

Anyone wants to help me on this? I just want to enter negative number based on the current code above. Please help...