Number of particles, given by $n = -D \frac{n_2 - n_1}{x_2 - x_1}$, is crossing a unit area perpendicular to $x$-axis in unit time, where $n_1$ and $n_2$ are number of particles per unit volume for the value of $x$ meant to $x_2$ and $x_1$. Find dimensions of $D$, called as diffusion constant.