Sum in an ode45?

I have the following differnetial equation

dy(1) = a*y(1)/N

where N is the sum of all the Y(i) at a given moment of time, not just at the beginning. How can I input this into ode45?

[Jan]

What are "all Y(i)"?

[Walter Roberson]

What does y(1) indicate in this context? Evaluation at y = 1 ? The first of set of subscripted variables?