On June 25, 2006, Warren Buffett (then the world's richest person, estimated worth of US$62 billion as of April 16, 2008) pledged to give the foundation approximately 10 million Berkshire Hathaway Class B shares spread over multiple years through annual contributions, with the first year's donation of 500,000 shares being worth approximately US$1.5 billion. Buffett set conditions so that these contributions do not simply increase the foundation's endowment, but effectively work as a matching contribution, doubling the Foundation's annual giving: "Buffett's gift came with three conditions for the Gates foundation: Bill or Melinda Gates must be alive and active in its administration; it must continue to qualify as a charity; and each year it must give away an amount equal to the previous year's Berkshire gift, plus an additional amount equal to 5 percent of net assets. Buffett gave the foundation two years to abide by the third requirement." The Gates Foundation received 5% (500,000) of the shares in July 2006 and will receive 5% of the remaining earmarked shares in the July of each following year (475,000 in 2007, 451,250 in 2008). In July 2013, Buffet announced another donation of his company's Class B, this time in the amount worth $2 billion, is going to the Bill and Melinda Gates Foundation.

What was warren Buffett's estimated net worth in 2008