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