$S_1:$  Median of a continuous frequency distribution, is given by the formula 
Median  $=l+\left(\frac{\left(\frac{n}{2}\right)-C_f}{f}\right)\times h,$ where $\mathcal{l}=$  lower limit of median class
$n=$  total frequency 
$f=$  frequency of the median class
$h=$ width of the median class
$c_f=$ cumulative frequency of the class preceding the median class
$S_2:$  Let  ${\overline{x}}_1$ and ${\overline{x}}_2$  be the respective means and ${\sigma }_1,{\sigma }_2$  be the respectively standard deviation of the two given series. Let ${\overline{x}}_{12}$  be the combined mean. Also $D_1=({\overline{x}}_1-{\overline{x}}_{12})$  and $D_2=({\overline{x}}_2-{\overline{x}}_{12})$  then combined standard deviation  $={\sigma }_{12}=\sqrt{\frac{n_1({\sigma }_1^2+D_1^2)+n_2({\sigma }_2^2+D_2^2)}{n_1^2+n_2^2}}$. Then the number of statement which are true is_______