A man wants to accumulate a sum of at least $500,000 by the end of 20 years. He plans to deposit $x yearly into an account that pays 12% interest compounded monthly, for the first 10 years, and $2x yearly into the same account for the next 10 years. He intends to make the first deposit at the beginning of year 1. Find the least value of x. Give your answer to the nearest integer.