The best place to get this number is from your originally-filed 2016 federal tax return. Don't have a copy of last year's return? You can order a transcript from the IRS. Go to their Get Transcript site, choose Get Transcript Online, and follow the directions (if you don't already have an account, you'll need to set one up).
See below for more details.
https://ttlc.intuit.com/replies/3301618
