"When no more circles can be inscribed, the distance between the first and the last inscribed circles is measured."

What is the definition of the distance between two circles?

As usual, min (dist(P1, P2)), where point P1 lies on the 1st circle, and point P2 lies on the 2nd circle

i.e. the difference of the distance between centers of these circles and the sum of their radiuses.

D=sqrt((c1.x-c2.x)^2+(c1.y-c2.y)^2) - r1 - r2.