How to sum attributes of a Eigen vector of class

I have declared a vector of a class I created like this:

typedef Eigen::Vector<Trade, Dynamic> VectorT;

And I am trying to sum a rolling window of trade.amount

Before switching from std::vector to Eigen::Vector I did this

for (size_t i = 30; i < trades.size(); ++i) {    double cum_buy_qty = 0;    for (size_t k = i - 30; k <= i; ++k) {        cum_buy_qty += trades[k].buyQuantity;}}

Now I am slicing the vector like that:

for (size_t i = 30; i < trades.rows(); ++i) {    VectorT slice = trades(seq(i - 30, i));}

But I don't know I could sum the attribute buyQuantity for each Trade in my new slice vector.

Any idea please?