D. Minimum number of steps

time limit per test1 second

memory limit per test256 megabytes

inputstandard input

outputstandard output

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 109?+?7.

The string "ab" appears as a substring if there is a letter 'b' right after the letter 'a' somewhere in the string.

Input

The first line contains the initial string consisting of letters 'a' and 'b' only with length from 1 to 106.

Output

Print the minimum number of steps modulo 109?+?7.

Examples

Input

ab

Output

1

Input

aab

Output

3

Note

The first example: "ab" ?→? "bba".

The second example: "aab" ?→? "abba" ?→? "bbaba" ?→? "bbbbaa".

Print the minimum number of steps modulo 109?+?7.

题意：给你一个只含'a'和'b'的串,让你把所有"ab"变为"bba"，直到串中不含"ab"的子串,输出共变了多少次；

思路:仔细看提示的样例，可以发现最后的串中'a'的数量不变，那是不是可以看作把当前串的最后一个'a'移动到当前串中最后一个'b'后面，让ans加上移动的次数，并且'a'每移动一次，'b'的个数加一，从尾到头跑一遍，输出ans即可；

#include<iostream>

#include<string.h>

using namespace std;

const int mod = 1e9 + 7;

int main()

{

 long long cnt = 0;

 long long ans = 0;

 char a[1000005];

 scanf("%s", a);

 int len = strlen(a);

 for (int i = len-1; i>=0; i--)

  if (a[i] == 'b')

   cnt++;

  else

  {

   ans=(ans+cnt)%mod;

   cnt= (cnt+(cnt%mod))%mod;

  }

 cout << ans << endl;

 return 0;

}