The packing efficiency of the face centred cubic $\left(\mathrm{fcc}\right)$, body centred cubic $\left(\mathrm{bcc}\right)$ and simple/primitive cubic $\left(\mathrm{pc}\right)$ lattices follows the order

A two-dimensional solid pattern formed by two different atoms $\mathrm{X}$ and $\mathrm{Y}$ is shown below. The black and white squares represent atoms $\mathrm{X}$ and $\mathrm{Y}$, respectively. The simplest formula for the compound based on the unit cell from the pattern is