A format for expressing an ordered list of integers is to use a comma separated list of either

 individual integers
 Or a range of integers denoted by the starting integer separated from the end integer in the range by a dash, '-'. (The range includes all integers in the interval including both endpoints)
 The range syntax is to be used only for, and for every range that expands to more than two values.
Example

The list of integers:

 -6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20
Is accurately expressed by the range expression:

 -6,-3-1,3-5,7-11,14,15,17-20
(And vice-versa).



Task

Expand the range description:

  -6,-3--1,3-5,7-11,14,15,17-20 

Note that the second element above,
is the range from minus 3 to minus 1. 



Related task

   Range extraction


import parseutils, strutils
 
proc expandRange(input: string): string =
  var output: seq[string]
  for range in input.split(','):
    var sep = range.find('-', 1)
    if sep > 0: # parse range
      var first = -1
      if range.substr(0, sep-1).parseInt(first) == 0:
        break
      var last = -1
      if range.substr(sep+1).parseInt(last) == 0:
        break
      for i in first..last:
        output.add($i)
    else: # parse single number
      var n = -1
      if range.parseInt(n) > 0:
        output.add($n)
      else:
        break
  return output.join(",")
 
echo("-6,-3--1,3-5,7-11,14,15,17-20".expandRange)