Two blocks of wood of masses $m_1$ and $m_2$ and each of specific gravity 0.5 are submerged at a depth of $h_1$ and $h_2$ in a vessel $\left(h_2>h_1\right)$ filled with water, which is accelerated upwards with an acceleration $g / 2$. The difference in time taken by the blocks to reach the surface, when released with zero velocity is -