（9991*1999.1999+9991.9991*1999）/2.0002
=（9991*1999*1.0001+9991*1.0001*1999）/2.0002
=1.0001（9991*1999+9991*1999）/2.0002
=1.0001*2*9991*1999/2.0002
=9991*1999
=19972009