Every U.S President We Have Had Has Been Mixed Race, Correct ?

After all there are many people here on yahoo answers who believe that nobody is in the world is racially pure. So that must mean that George W. Bush, Bill Clinton, Ronald Reagan, Jimmy Carter, Richard Nixon, Lyndon Johnson, JFK, etc are all Mixed Race. So it is false to say that Obama is America's first Mixed Race president if everybody in the U.S and the world is Mixed Race.