Chebyshev lower bound for a sum of two beta variables

Let $X_1$ and $X_2$ be i.i.d. continuous random variables with the probability density function

$$f(x)=\begin{cases}6x(1-x), & 0<x<1,\ 0, & \text{otherwise.}\end{cases}$$

Using Chebyshev's inequality, the lower bound of $P\left(|X_1+X_2-1|\le\frac12\right)$ is