CF1084C The Fair Nut and String

The Fair Nut found a string $ s $ . The string consists of lowercase Latin letters. The Nut is a curious guy, so he wants to find the number of strictly increasing sequences $ p_1, p_2, \ldots, p_k $ , such that: 1. For each $ i $ ( $ 1 \leq i \leq k $ ), $ s_{p_i} = $ 'a'. 2. For each $ i $ ( $ 1 \leq i < k $ ), there is such $ j $ that $ p_i < j < p_{i + 1} $ and $ s_j = $ 'b'. The Nut is upset because he doesn't know how to find the number. Help him. This number should be calculated modulo $ 10^9 + 7 $ .

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In the first example, there are $ 5 $ possible sequences. $ [1] $ , $ [4] $ , $ [5] $ , $ [1, 4] $ , $ [1, 5] $ . In the second example, there are $ 4 $ possible sequences. $ [2] $ , $ [3] $ , $ [4] $ , $ [5] $ . In the third example, there are $ 3 $ possible sequences. $ [1] $ , $ [3] $ , $ [4] $ .