A system consists of two reactors, connected by a valve. The first reactor $(R1)$ contains an ideal gas $A$ of volume $5\: L$ and the second reactor $(R2)$ has an ideal gas $B$ of volume $10\: L$. Initially, the valve is closed and pressure $P$ in $R1$ and $R2$ are $9$ and $6 \textit{ atm}$, respectively. Later, when the valve is opened, the system reaches equilibrium. If the temperature $T$ of both the reactors is maintained constant, the final equilibrium pressure in $\textit{atm}$ of the system is _____________