class ListNode:
  def __init__ (self, value, nxt):
    self.value = value
    self.nxt = nxt
  def set_value (self, value):
    self.value = value
  def set_nxt (self, nxt):
    self.nxt = nxt

class SinglyList:
  def __init__ (self):
    self.first = ListNode(0, -1)
  def __len__ (self):
    lenth = 1
    elem = self.first
    while elem.nxt != -1:
      lenth += 1
      elem = elem.nxt
    return lenth
  def insert_after (self, value, pos):
    elem = self.first
    for i in range(pos):
      if self.first.nxt == -1:
        return -1
    elem = self.first.nxt