Hey guys! I need help in completing the proof.

The proof starts like this

$\displaystyle E[X]=E[X^+]-E[X^-]=\int_{0}^{\infty} xdF_x^+(x)\, dx - \int_{0}^{\infty} xdF_x^-(x)\,dx$

$\displaystyle = \int_{-\infty}^{\infty} xdF_x(x)\, dx

$

I cant find the reason for the last line. Please give me a clue.