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