Let \( a_1 = 1 \) and for \( n \ge 1 \), \( a_{n+1} = \frac{1}{2} a_n + \frac{n^2 - 2n - 1}{n^2 (n+1)^2} \). Then \( \left| \sum_{n=1}^{\infty} \left( a_n - \frac{2}{n^2} \right) \right| \) is equal to _________.