$\mathbb{E}X^2=\sum_{k=1}^\infty (2k-1)\mathbb{P}(X\ge k)$

Question

I want to prove the following but don't know how:

$\mathbb{E}X^2=\sum_{k=1}^\infty (2k-1)\mathbb{P}(X\ge k)$

I proved $\mathbb{E}X=\sum_{k=1}^\infty\mathbb{P}(X\ge k)$ before but I don't know if that helps here.