1999+199.9+19.99+1.999+0.1999
=2000-1+200-0.1+20-0.01+2-0.001+0.2-0.0001
=2000+200+20+2+0.2-（1+0.1+0.01+0.001+0.0001）
=2222.2-1.1111
=2221.089