|1/2011-1/2010|+|1/2010-1/2009|+|1/2009-1/2008|+...+|1/3-1/2|+|1/2-1|
=-1/2011+1/2010-1/2010+1/2009-1/2009+1/2008-.-1/3+1/2-1/2+1
=1-1/2011
=2010/2011