About 40 billionaires in the United States have pledged to give at least half of their wealth to charity.
The promise is part of a campaign by investor Warren Buffett and Microsoft founder Bill Gates.
Billionaires who have joined the campaign include the New York mayor, Michael Bloomberg, entertainment executive Barry Diller and Oracle co-founder, Larry Ellison.
The Giving Pledge campaign was launched in June, to convince other American billionaires to give away most of their fortune, either during their lifetime, or after their death.
