$2NO\left(g\right)+C{l}_2\left(g\right)\underset{ }{\rightleftharpoons }2NOCl\left(s\right)$ This reaction was studied at   $-{10}^{0}C$ and the following data was obtained  
Run          ${\left[NO\right]}_0$       ${\left[C{l}_2\right]}_0$    $r_0$
1           0.10          0.10         0.18
2           0.10          0.20         0.35
3           0.20          0.20         1.40
${\left[NO\right]}_0$ and ${\left[C{l}_2\right]}_0$  are the initial concentrations and $r_0$  is the initial reaction rate. The overall order of the reaction is ____________. (Round off to the Nearest Integer).