Get current input value and add or subtract 1 to generate the new input value?

[amandaNHT]

I've got something like this...

document.getElementById('bump23id').value= (?)

...for a button.

There is an input field (a text field) with the name/id "bump23id".

When someone clicks the button, I want the "bump23id" field to go up one number (like if it's already at 3, it will go to 4)

I also want to create a second button just like the first that will make it go down one number.

How do I make it get the current value of the input field and then add or subtract 1 from the number to create the new value?

[xelawho]

<input type="button" value="up" onclick="Number(document.getElementById('bump23id').value++)" >

[amandaNHT]

Thanks!