On line 3, if you multiply $$L(p \mid \boldsymbol y) = f(\boldsymbol y \mid p) = \prod_{i=1}^n \binom{n}{y_i} p^{y_i} (1-p)^{n-y_i}.$$
why does the power on the (1-p) not go to $n^2 - \sum_{i=1}^n Y_i$ since youre multiplying (n-yi) n times?
Thank you!