21=2^1=2,
22=2^2=4,
23=2^3=8,
24=2^4=16,
25=2^5=32,
26=2^6=64,
27=2^7=128,
28=2^8=25621=2^1=2,22=2^2=4,23=2^3=8,24=2^4=16,25=2^5=32,26=2^6=64,27=2^7=128,28=2^8=256