Comparing 2 Input's

I'm having trouble comparing 2 input boxes, using getElementById.

I have 81 hidden inputs (id's b0 through b80) and 81 input boxes (id's a0-a80). (Its just sudoku, so theres no security threat with them being viewable in the source)

I'm using this code (ill worry about the for loop later)

var solved = true;
if(document.getElementById(a0).value != document.getElementById(b0).value) {
solved = false
alert("not solved"); // debug
}
But it doesnt seem to be working. Im getting the error a0 is not defined. I know for sure that I have an input box with id=a0.