CGMO 2010 Problem 8

Determine the least odd number $a > 5$ satisfying the following conditions: There are positive integers $m_1,m_2, n_1, n_2$ such that $a=m_1^2+n_1^2$ , $a^2=m_2^2+n_2^2$ , and $m_1-n_1=m_2-n_2.$