does this converge or diverge, if it converges, to what sum.

(2/3) - (4/9) + (8/27) - (16/81)

so a(n) = (-1)^n * 2^(n+1)/3^(n+1)

i know its convergent by alternating series test, but how can i figure out what sum it converges to without Sn formula?