Let for a differentiable function \( f : (0, \infty) \to \mathbb{R} \), \( f(x) - f(y) \ge \log_e\!\left(\frac{x}{y}\right) + x - y, \, \forall \, x, y \in (0, \infty) \). Then \( \displaystyle \sum_{n=1}^{20} f'\!\left(\frac{1}{n^2}\right) \) is equal to ____________.