Let \( f : [0,1] \to \mathbb{R} \) be a twice differentiable function in \( (0,1) \) such that \( f(0) = 3 \) and \( f(1) = 5 \). If the line \( y = 2x + 3 \) intersects the graph of \( f \) at only two distinct points in \( (0,1) \), then the least number of points \( x \in (0,1) \), at which \( f''(x) = 0 \), is __________.
