Concordance probability for independent pairs

Let $(X_1, Y_1), (X_2, Y_2), \dots, (X_n, Y_n)$, $n \ge 2$, be a random sample from a continuous bivariate distribution with joint distribution function $F_{X,Y}$. Further, $F_X$ and $F_Y$ are the marginal distribution functions of $X$ and $Y$, respectively. If $F_{X,Y}(x, y) = F_X(x)F_Y(y)$ for all $(x, y)$, then, for any two independent pairs $(X_i, Y_i)$ and $(X_j, Y_j)$, $P[(X_i - X_j)(Y_i - Y_j) > 0]$ equals