int birthday(vector<int> s, int d, int m) {
    assert(s.size() > 0 && d > 0 && m > 0);
    int sum = accumulate(s.cbegin(), s.cbegin() + m, 0);
    int r = 0;
    
    for (int i = 0, e = s.size() - m; i < e; ++i)
    {
        if (sum == d)
        {
            ++r;
        }
        
        sum += s[i + m];
        sum -= s[i];
    }
    
    if (sum == d)
    {
        ++r;
    }
    
    return r;
}