s=1+1/3+1/3^2+....................+1/3^1999
s/3=1/3+1/3^2+1/3^3+........+1/3^2000
s-s/3=1 - 1/3^2000
s(1-1/3)=1 - 1/3^2000
s=3(1-1/3^2000)/2
