template <class T>
  T median (std::vector<T> v) {
      typedef typename vector<T>::size_type vec_sz;
      vec_sz size= v.size();
      if (size == 0)
        throw domain_error("median of an empty vector");
      sort(v.begin(), v.end());
      vec_sz mid = size / 2;
      return size % 2 == 0 ? (v[mid] + v[mid - 1]) / 2 : v[mid];
}