Continuous random variable

Suppose your goal is to describe the continuous random variable using the probability density function. You have conducted a precise experiment and, as a result, you have got the following distribution function

$F(x)= \left\{\begin{matrix} 0, x< 0\\ \frac{x^{2}}{4},0\leq x\leq 2 \\ 1, x>2 \end{matrix}\right.$

What is the mathematical expression for the probability density function on the interval $0\leq x\leq 2$?