1-1/2=1/2
1-1/2-1/4=1/4
1-1/2-1/4-1/8=1/8
1-1/2-1/4-1/8-1/16=1/16
.
1-1/2-1/4-1/8-1/16-1/32-1/64-1/128-1/256-1/512=1/512