2019 AIME I Problem 10

For distinct complex numbers z_{1}, z_{2}, \ldots, z_{673}, the polynomial

\left(x-z_{1}\right)^{3}\left(x-z_{2}\right)^{3} \cdots\left(x-z_{673}\right)^{3}

can be expressed as x^{2019}+20 x^{2018}+19 x^{2017}+g(x), where g(x) is a polynomial with complex coefficients and with degree at most 2016 . The value of

\left|\sum_{1 \leq j<k \leq 673} z_{j} z_{k}\right|

can be expressed in the form \frac{m}{n}, where m and n are relatively prime positive integers. Find m+n.