def solve(input):
  input = sorted(input)
  result = []
  while len(input) > 0:
    base = input.pop(0)
    while len(input) > 0 and input[0][0] <= base[1] + 1:
      base = [base[0], max(base[1], input[0][1])]
      input.pop(0)
    result.append(base)
  return result

print(solve([[1, 3], [4, 6], [5, 8], [12, 17], [20, 23], [6, 9], [22, 25], [13, 15]]))