# Why is this simple partial derivative correct?

This example is taken from Geoffrey Hinton's Coursera on neural networks (seen at 13:15).

$$E=\sum_ip_i(t-y_i)^2,\\ \frac{\partial E}{\partial y_i}=p_i(t-y_i)$$

When I calculate this partial derivative I get:

$-2p(t-y)$

Does the sigma change how we calculate derivatives?