Four identical particles each of mass $m$ are kept at the four corners of a square of side $a$. If one of the particles is removed, the shift in the position of the centre of mass is

A system consists of two particles of masses $m_1$ and $m_2$. If the particle of mass $m_1$ is moved towards the centre of mass through a distance $d$, then the distance the second particle should be moved, so as to keep the centre of mass at the same position is