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. What did Melinda Gates set so the contributions work as a matching contribution?