The AGI you should use to sign your 2018 return can be found on the following lines of your 2017 return (round this amount to the nearest whole dollar):
To retrieve your original AGI from your previous year's tax return (or from the original return if you filed an amended return) you may do one of the following:
If your filing status changed from the previous year to "Married Filing Joint", then each taxpayer will use their individual original AGI from their respective prior year tax returns.
If the change is from "Married Filing Joint", then both taxpayers will use the same original AGI from the prior year's joint tax return.
If you have never filed before, or if you filed last year using an ITIN and you have a Social Security Number this year, choose I am a first-time filer or I did NOT file a 2016 tax return on the prior year information screen.
If you are married filing jointly and only one spouse filed last year, enter the prior year AGI for that spouse and zero (0) for the spouse who has never filed before.
If you filed Form 1040X to amend your return last year, be sure to use the AGI amount from the originally-filed return. Do NOT use the amount from the amended return.