Activity of a radioactive sample is $R_1$ at a time $t_1$ and $R_2$ at a time $t_2$. Its half life period is $T$. The number of atoms that have disintegrated in the time interval $\left(t_2-t_1\right)$ is equal to $\frac{n\left(\mathrm{R}_1-\mathrm{R}_2\right) \mathrm{T}}{\ln 4}$. Then ' $n$ ' is equal to