Two blocks of masses $m_1=2 kg$ and $ m_2=4kg$ are moving in the same direction with speeds $V_1=6 m/s$ and $V_2=3 m/s$, respectively on a frictional less surface. An ideal spring with spring constant $k= 30000\;n/m$ is attached to the bakside of $m_2.$ Then the maxiimum compression of the spring after collision will be