import sys
sys.setrecursionlimit(500000)
def scc(edge):
  n = len(edge)
  redge = [[] for i in range(n)]
  for u in range(n):
    for v in edge[u]:
      redge[v].append(u)
  seen = [0] * n
  order = []
  group = [0] * n

  def dfs(u):
    seen[u] = 1
    for v in edge[u]:
      if seen[v] == 0:
        dfs(v)
    order.append(u)

  def rdfs(u, label):
    seen[u] = 1
    group[u] = label
    for v in redge[u]:
      if seen[v] == 0:
        rdfs(v, label)

  for u in range(n):
    if seen[u] == 0:
      dfs(u)

  seen = [0] * n
  label = 0
  for u in reversed(order):
    if seen[u] == 0:
      rdfs(u, label)
      label += 1
  
  return group


from collections import Counter
n = int(input())
m = 10
e = [[] for _ in range(m)]
ls = [list(map(lambda x: int(x)-1, input().split())) for _ in range(n)]
ind = [0] * m
for h, w in ls:
  e[h].append(w)
  ind[w] += 1
g = scc(e)
cnt = [0, 0]
for i in range(m):
  if abs(len(e[i]) - ind[i]):
    if abs(len(e[i]) - ind[i]) > 1:
      print(0)
      exit()
    if len(e[i]) > ind[i]:
      cnt[0] += 1
    else:
      cnt[1] += 1
if cnt[0] != cnt[1] or max(cnt) > 1:
  print(0)
  exit()
elif cnt[0]:
  print(1)
else:
  print(max(Counter(g).values()))