cf 805D

#include<stdio.h>
#include<string.h>

int main()
{
    char s[1000009];
    scanf("%s", s);

    int i, j, c = 0, k = 0;
    for(i = strlen(s) - 1; i >= 0; i--)
    {
        if(s[i]=='b') c++;
        else
        {
            k += c;
            c *= 2;
            k %= 1000000007;
            c %= 1000000007;
        }
    }
    printf("%d\n", k);
}