Shortest Distance around a volcano

I'm reviewing for the final term test and came across this problem I couldn't solve from a test and its been a while since the unit:

Two divers are directly opposite each other along the edge of an underwater volcano (or a cone). The volcano is 20 feet in diameter and 10 feet high. What is the shortest distance one diver would swim to reach the other.

I've drawn the cone unfolded as a sector of the circle. Would the radius be equal to VA (line segment vertex to diver a) and VB (line segment vertex to diver b)? What would t be?

D = Sqrt(VA^2 + VB^2 - 2*VA*VB*cos(t))

Thanks