Let $A=\{1,2,3, \ldots 20\}$. Let $R_1$ and $R_2$ two relation on A such that $\mathrm{R}_1=\{(\mathrm{a}, \mathrm{b}): \mathrm{b}$ is divisible by a$\}$ $\mathrm{R}_2=\{(\mathrm{a}, \mathrm{b}): \mathrm{a}$ is an integral multiple of b$\}$. Then, number of elements in $R_1-R_2$ is equal to