Conditional Expectation I

Consider the following joint pdf:

$$\begin{aligned} f_{X_1, X_2}(x_1, x_2) &= \begin{cases} c(1 - x_2) & 0 \leq x_1 \leq x_2 \leq 1 \\ 0 & \text{otherwise} \end{cases} \end{aligned}$$

where $c$ is a constant such that $f_{X_1, X_2}$ is a valid joint pdf. Compute $\mathbb{E}[X_1]$.