Let \( S_1 = \{100, 105, 110, 115, \dots\} \) and \( S_2 = \{100, 95, 90, 85, \dots \} \) be two series in arithmetic progression. If \(a_k\) and \(b_k\) are the k-th terms of \(S_1\) and \(S_2\), respectively, then \( \sum_{k=1}^{20} a_k b_k \) equals ________