Well, your answer is based upon the assumption that the man can turn sideways while running.
If he is facing 'forward' the minimum raindrops is 12,000,000
If he can run while sideways, then the minimum would be 1,000,000
How did you get an optimal*2 of 3,000,000?
Should optimal *2 be either 2,000,000 or 24,000,000