Let’s also not ignore whether these men served one or two terms.  Here are the one term Presidents:

John Adams

John Quincy Adams

Martin Van Buren

James K. Polk

Franklin Pierce

James Buchanan

Rutherford B. Hayes

William Howard Taft

Herbert Hoover

Jimmy Carter

George H. W. Bush

Do they get half credit?

Others died in office.  You may wish to apportion their frequency of appearance accordingly.  Here is a list of days served.

William Henry Harrison       31 days

Zachary Taylor                      491 days

Abraham Lincoln                   1,503 days

James A. Garfield                   199 days

William McKinley                  1,654 days

Warren G. Harding                881 days

Franklin Roosevelt                4,422 days

John F. Kennedy                    1,036 days

The number of days is probably an important factor.