A12185. 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 $10^9 + 7$ .

The first line contains the string $s$ ( $1 \leq |s| \leq 10^5$ ) consisting of lowercase Latin letters.

In a single line print the answer to the problem — the number of such sequences $p_1, p_2, \ldots, p_k$ modulo $10^9 + 7$ .

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. $[1]$ , $[3]$ , $[4]$ .