A litre of buffer solution contains 0.1 mole of each of NH$_3$ and NH$_4$Cl. On the addition of 0.02 mole of HCl by dissolving gaseous HCl, the pH of the solution is found to be _____________ $\times$ 10$^{-3}$ (Nearest integer)

[Given : $\mathrm{pK_b(NH_3)=4.745}$

$\mathrm{\log2=0.301}$

$\mathrm{\log3=0.477}$

$\mathrm{T=298~K]}$