<image> Given the universal set \( U = \mathbb{R} \), set \( M = \{ x \mid -1 \leq x \leq 3 \} \) and set \( N = \{ x \mid x = 2k - 1, k = 1, 2, \ldots \} \). The Venn diagram of their relationship is shown in the figure. The number of elements in the shaded region is _____.