A weightless rod of length `2l` carries two equal masses `\'m\'` , one secured at lower end `A` and

A weightless rod of length `2l` carries two equal masses `\'m\'` , one secured at lower end `A` and the other at the middle of the rod at `B`. The rod can rotate in vertical plane about a fixed horizontal axis passing through `C`. What horizontal velocity must be imparted to the mass at `A` so that it just completes the vertical circle.