Dice Rolling Covariance

A fair, six-sided die is rolled 36 times. What is the covariance between the number of 1s rolled and the number of 2s rolled?

In other words, if $N_1$ is the number of 1s and $N_2$ is the number of 2s, what is $E(N_1N_2)-E(N_1)E(N_2)?$