Generally, many of us recommend putting the box 2a value in there...as long as box 2a has an actual non-zero value in it.
If box 2a is empty, or marked as being "undetermined"....then as long as you didn't put make an "After-tax" contributions to that type of retirement account, then you could likely just enter the box 1 value into box 16.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
