The mean and standard deviation of $10$ observations $x_1, x_2, x_3.....x_{10}$ are $\overline{x}$ and $\sigma$ respectively. Let $10$ is added to $x_1, x_2.....x_9$ and $90$ is subtracted from $x_{10}.$ If still, the standard deviation is the same, then $x_{10}-\overline{x}$ is equal to