If the standard deviation of 10 observations $x_1,x_2,\mathrm{.....},x_{10}$ is 4 and that of another set of 10 observations $y_1,y_2,\mathrm{.......}{y}_{10}$ is 3 and also $X_i=\left(x_i-\overline{x}\right)\left(y_i-\overline{y}\right)$. $\overline{x}$ is mean of all  $x_i$’s and  $\overline{y}$ is mean all  $y_i$’s  and  $\sum _{i=1}^{10}{X}_i=80$. Then standard deviation of observations $x_1-y_1,x_2-y_2,\mathrm{......},x_{10}-y_{10}$ is