Kendall's tau from the concordant count: $(C-D)/\binom n2$

$(X_i,Y_i)$, $i=1,\dots,9$ is a random sample from bivariate population where all $X_i$'s are distinct and all $Y_i$'s are distinct, $i=1,\dots,9$. If the number of concordance pairs is 9, the Kendall's sample correlation coefficient is: