B. Minimum number of stepstime limit per test1 secondmemory limit per test256 megabytesinputstandard inputoutputstandard outputWe have a string of letters 'a' and 'b'. We want to perform some operations on it. On each step we choose one of substrings "ab" in the string and replace it with the string "bba". If we have no "ab" as a substring, our job is done. Print the minimum number of steps we should perform to make our job done modulo 109 + 7.The string "ab" appears as a substring if there is a letter 'b' right after the letter 'a' somewhere in the string.InputThe first line contains the initial string consisting of letters 'a' and 'b' only with length from 1 to 106.OutputPrint the minimum number of steps modulo 109 + 7.ExamplesInputabOutput1InputaabOutput3NoteThe first example: "ab"  →  "bba".The second example: "aab"  →  "abba"  →  "bbaba"  →  "bbbbaa".
Inputab
Output1
['combinatorics', 'greedy', 'implementation', 'math', '*1400']