CMO 1975 Problem 2

A sequence of numbers $a_1, a_2, a_3, ...$ satisfies

(i) $a_1 = \frac{1}{2}$ (ii) $a_1+a_2 + \cdots + a_n = n^2 a_n \ (n \geq 1)$

Determine the value of $a_n \ (n \geq 1)$ .