# Finding Probability Using CDF

How do I find PMF if I have CDF? For example:

$$F_X (x)=\left\{\begin{array}{cc}0,&\mbox{ if }x\leq -1 \\0.2, & \mbox{ if } -1\leq x < 0 \\0.7, & \mbox{ if } 0\leq x < 1 \\1, & \mbox{ if } x \geq 1 \end{array}\right.$$

f(x) = $\Delta F(x)/\Delta x$