B

#include<iostream>#include<vector>#include<string>#include<set>#include<map>#include<algorithm>#include<queue>#include<list>#include<stack>#include<cstdio>#include<fstream>#include<numeric>#include<functional>#include<utility>#include<memory>#include<iomanip>using namespace std;string s;long long int Mod = 1000000000 + 7;long long int power(long long int a, long long int b){long long int res = 1;long long int k = a;for (; b; b >>= 1, k = (k*k) % Mod){if (b & 1) res = (res*k) % Mod;}return res;}int main(){while (cin >> s){long long int numa = 0;long long int total = 0;for (int i = 0; i < s.size(); i++){if (s[i] == 'a') numa++;else{long long int t = 2;total = (total + power(t, numa) - 1) % Mod;}}cout << total << endl;}return 0;}