1/2-1/4+1/8-1/16+1/32-1/64+1/128-1/256
=(1/2-1/4)+(1/8-1/16)+(1/32-1/64)+(1/128-1/256)
=1/4+1/16+1/64+1/256
=64/256+16/256+4/256+1/256
=85/256