C题
#include<stdio.h>
#include<iostream>
#include<algorithm>
#include<cmath>
#include<cstring>
#include<string.h>
#include<queue>
#include<stack>
#include<list>
#include<map>
#include<set>
#include<vector>
using namespace std;
typedef long long int ll;
typedef unsigned long long ull;
const int maxn =1e5+5;
const int maxm=10000;
const int mod =1e9+7;
const int INF=0x3f3f3f3f;
const double eps=1e-8;
int main()
{
int a=0;
ll sum=1;
string s; cin>>s;
for(int i=0;i<s.length();i++)
{
if( s[i]=='a' ) a++;
else if (s[i]=='b'){
sum=(sum*(a+1))%mod;
a=0;
while(s[i]!='a'&&i<s.length())
i++;
i--;
}
}
sum*=(a+1);
cout<<(sum-1)%mod<<endl;
}