2002又1/2-2001又1/2+2000又1/2-1999又1/3+…+2又1/2-1又1/3
=(2002-2001+2000-1999+.+2-1)+(1/2-1/2+1/2-1/2+.+1/2-1/2)
=1*2002/2
=1001