If each term of a geometric progression \( a_{1},a_{2},a_{3},\ldots \) with \( a_{1}=\dfrac{1}{8} \) and \( a_{2}\neq a_{1} \), is the arithmetic mean of the next two terms and \( S_{n}=a_{1}+a_{2}+\cdots +a_{n} \), then \( S_{20}-S_{18} \) is equal to:
