The ordinates of a one-hour unit hydrograph for a catchment are given below:
$$\begin{array}{|l|l|l|l|l|l|l|l|l|}
\hline \text{t(hour)} & 0 & 1 & 2 & 3 & 4 & 5 & 6 & 7 \\
\hline \text{Q(m}^3 \text{/s)} & 0 & 9 & 21 & 18 & 12 & 5 & 2 & 0 \\
\hline
\end{array}$$
Using the principle of superposition, a $D$-hour unit hydrograph for the catchment was derived from this one-hour unit hydrograph. The ordinates of the $D$-hour unit hydrograph were obtained as $3 \mathrm{~m}^{3} / \mathrm{s}$ at $t=1$ hour and $10 \mathrm{~m}^{3} / \mathrm{s}$ at $t=2$ hour. The value of $D$ (in integer) is ______________.