For an object to complete a vertical circle, the necessary centripetal force at the highest point is provided by its weight and the normal reaction from the track. The equation of motion is $N + mg = \frac{mv^2}{R}$. For the minimum speed at the highest point, the normal reaction $N$ approaches zero ($N \ge 0$). Thus, $mg = \frac{mv^2}{R}$, which simplifies to $v = \sqrt{gR}$ .