If \( X \) is a continuous random variable whose probability density function is given by
\[
f_X(x) =
\begin{cases}
c x^3 + 0.25, & \text{for } 0 \leq x \leq 1,\ c \in \mathbb{R} \\
0, & \text{elsewhere}
\end{cases}
\]
Then the value of \( c \) is ________________ (in integer).