Roth Withdrawal question

I have a Roth IRA that I funded years ago and now I took a qualified withdrawal. The fund has grown to more than double what I put in. I withdrew more than I put in, but not all of the fund. Now TurboTax wants me to enter how much of my original investment is left. It says subtract what I took out from my original investment. That would be a negative number. Should I enter $0 or a negative number? It doesn't say enter $0 if you have withdrawn more than the original investment. I'm guessing to enter $0 but I don't know what it is getting at or even why it is asking. Anybody??