Let the circles \( C_1: |z| = r \) and \( C_2: |z - 3 - 4i| = 5, \ z \in \mathbb{C}, \) be such that \( C_2 \) lies within \( C_1 \). If \( z_1 \) moves on \( C_1, z_2 \) moves on \( C_2 \) and \( \min |z_1 - z_2| = 2 \), then \( \max |z_1 - z_2| \) is equal to: