|1/2007-1/2006|+|1/2006-1/2005|+|1/2005-1/2004|-|1/2004-1/2007|=[-(1/2007-1/2006)]+ [-(1/2006-1/2005)]+ [-(1/2005-1/2004)]-(1/2004-1/2007)=-1/200+1/2006-1/2006+1/2005-1/2005+1/2004-1/2004+1/2007=0