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The state distribution amount is the value that is in box 2a.

BUT

if box 2a is empty, then it is the Federally-taxable amount of box 1  (which is usually just box 1, but can be lower if there were extra after-tax contributions you made to that retirement fund))

____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*