Please help to stupid goat.
My solution:

if AtSegm(A, B, CrossLine(PointsToLine(A, B), PerLine(PointsToLine(A, B), C)))
  then Writeln(NullOrNum(   Dist(CrossLine(PointsToLine(A, B), PerLine(PointsToLine(A, B), C)), C)    -len) : 0 : 2)
  else Writeln(NullOrNum(min(Dist(A, C), Dist(B, C))-len) : 0 : 2);

Writeln(NullOrNum(max(Dist(A, C), Dist(B, C))-len) : 0 : 2);

where
  AtSegm(A, B, C) - point locate on segment
  PerLine(L) - perpendicular of L
  NullOrNum(x) = max(0, x)
  Dist(A, B) - distantion between points A and B
  len - length of rope

Where may be bug?

Here there should be still even one "else" ...