99999/2+9999/2+999/2+99/2+9/2
=（100000-1）/2+(10000-1)/2+(1000-1)/2+(100-1)/2+(10-1)/2
=50000-1/2+5000-1/2-+500-1/2+50-1/2+5-1/2
=55555-5/2
=55552.5