Let $X=\{11,12,13,....,40,41\}$ and $Y=\{61,62,63,....,90,91\}$ be the two sets of observations. If $\overline x $ and $\overline y $ are their respective means and $\sigma^2$ is the variance of all the observations in $\mathrm{X\cup Y}$, then $\left| {\overline x + \overline y - {\sigma ^2}} \right|$ is equal to ____________.