Consider the given function \( f(x) \).
\[
f(x) = \begin{cases}
ax + b & \text{for } x < 1 \\
x^3 + x^2 + 1 & \text{for } x \geq 1
\end{cases}
\]
If the function is differentiable everywhere, the value of \( b \) must be __________ (rounded off to one decimal place).