参考答案：

#include<iostream>
#include<string>
using namespace std;
int main()
{
    string a;
    getline(cin, a);
    int sum = 0;
    while (a[0] != '.')
    {
        for (int i = 0; i < a.size(); ++i)
        {
            if (a[i] == 'a')
                ++sum;
        }
        getline(cin, a);
    }
    cout << sum;
    return 0;
}