1/4+1/8+1/16+1/32+1/64+1/128
=1/4+1/8+1/16+1/32+1/64+1/128+1/128-1/128
=1/4+1/8+1/16+1/32+1/64+1/64-1/128
=1/4+1/8+1/16+1/32+1/32-1/128
=1/4+1/8+1/16+1/16-1/128
=1/4+1/8+1/8-1/128
=1/4+1/4-1/128
=1/2-1/128
=63/128
S=1/4+1/8+1/16+1/32+1/64+1/128
2s=1/2+1/4+1/8+1/16+1/32+1/64
2s-s=S=1/2-1/128