Let \(a_1, a_2, a_3, \ldots, a_{10}\) be in G.P. with \(a_i \gt 0\) for \(i = 1, 2, \ldots, 10\) and S be the set of pairs \((r, k)\), \(r, k \in N\) (the set of natural numbers) for which
\(\begin{vmatrix} \log_e a_1^r a_2^k & \log_e a_2^r a_3^k & \log_e a_3^r a_4^k \\ \log_e a_4^r a_5^k & \log_e a_5^r a_6^k & \log_e a_6^r a_7^k \\ \log_e a_7^r a_8^k & \log_e a_8^r a_9^k & \log_e a_9^r a_{10}^k \end{vmatrix} = 0\).
Then the number of elements in S, is -