Covariance from a triangular density

Let $(X_1, X_2)$ have joint density $f(x_1, x_2) = c\,(1 - x_2)$ on the region $0 \le x_1 \le x_2 \le 1$ (and $0$ otherwise), where $c$ normalizes the density. Compute $\operatorname{Cov}(X_1, X_2)$.