Two blocks of masses $10 \mathrm{kg}$ and $4 \mathrm{kg}$ are connected by a spring of negligible mass and placed on a frictionless horizontal surface. An impulse gives a velocity of $14 \mathrm{m} {\mathrm{s}}^{-1}$ to the heavier block in the direction of the lighter block. The velocity of the centre of mass in $\mathrm{m} {\mathrm{s}}^{-1}$ is $2n$, then $n=$