Minimum number of steps time limit per test 2 second memory limit per test 256 megabytes We 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 1e9+7. The string “ab” appears as a substring if there is a letter ‘b’ right after the letter ‘a’ somewhere in the string.