Two cells are connected between points $A$ and $B$ as shown. Cell 1 has emf of $12 \mathrm{V}$ and internal resistance of $3 \mathrm{\Omega }$. Cell 2 has emf of $6 \mathrm{V}$ and internal resistance of $6 \mathrm{\Omega }$. An external resistor $R$ of $4 \mathrm{\Omega }$ is connected across $A$ and $B$. The current flowing through $R$ will be ______ $\mathrm{A}$.