a*b=（a-b）/ab＝(1/b)-(1/a)
2*1+3*2+4*3+……+2002*2001
=( 1-1/2)+(1/2-1/3)+1/3-1/4)+.+(1/2001-1/2002)
=1-(1/2002)
=2001/2002