The standard deviations of $x_i (i=1,2,\ldots,10)$ and $y_i (i=1,\ldots,10)$ are respectively $a$ and $b$. $\bar{x}$, $\bar{y}$ are the means. If $z_i = (x_i-\bar{x})(y_i-\bar{y})$ and $\sum_{i=1}^{10} z_i = c$, then the standard deviation of $(x_i-y_i), (i=1,2,\ldots,10)$ is