The ordinates of a one-hour unit hydrograph for a catchment are given below: 

$$ \begin{array}{|l|l|l|l|l|l|l|l|l|} \hline t(\text { hour }) & 0 & 1 & 2 & 3 & 4 & 5 & 6 & 7 \\ \hline Q\left(\mathrm{~m}^3 / \mathrm{s}\right) & 0 & 9 & 21 & 18 & 12 & 5 & 2 & 0 \\ \hline \end{array} $$

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 m3/s at t = 1 hour and 10 m3/s at t = 2 hour. The value of D (in integer) is _____________.