What was the population of the world at the time of the flood?

I understand that from the perspective of those who take the Bible literally, the global flood took place about 2400 B.C. and that the world began about 4000 B.C., giving 1600 years of human population growth. So, what was the estimated population at the time of the flood? In other words, how many people incurred the judgment of God in the flood? Please answer according to the perspective of those who take the Bible literally and historically.