Strings And Numbers...?

I'm having difficulties arithmetically manipulating form element values.

I've entered data into the form, and I fetch them using a js, as:

p7Left = Number(document.form1.elements["p7_left"].value);
p7Right = Number(document.form1.elements["p7_right"].value);
...
...
scoreLeft = Number(document.form1.elements["left_score"].value);
scoreRight = Number(document.form1.elements["right_score"].value);

Then I add these as follows:

scoreLeft = Number(scoreLeft + p&Left + ...... +);

This does what I want; without the operator 'Number' I get a concatination
of the various variables (as expected). Is there some way of globally
defining all variables as numbers instead of strings?