Two cells, having the same E.M.F. are connected in series through an external resistance $\mathrm{R}$. Cells have internal resistances ${\mathrm{r}}_1$ and ${\mathrm{r}}_2$ $\left({\mathrm{r}}_1>{\mathrm{r}}_2\right)$ respectively. When the circuit is closed, the potential difference across the first cell is zero. The value of $\mathrm{R}$ is