I am having trouble in solving the following problem;
If $X_1, X_2, . . . \in \mathbb{R}$ are exchangeable with $EX_i^2 < \infty$ then $E(X_1X_2) ≥ 0.$
What I know is that the definition of exchangeable sequence;
A sequence $X_1, X_2, . . . $is said to be exchangeable if for each $n$ and permutation $\pi$ of $\{1, . . . , n\}, (X_1, . . . , X_n)$ and $ (X_{\pi(1)}, . . . , X_{\pi(n)})$ have the same distribution.
How could I develop ideas from the above simple definition? Are there any hints available? Thanks in advance.