import itertools
n = int(input())
lr = []
for i in range(n):
  l,r = map(int,input().split())
  lr.append((l,r))
ans = 0
for res in itertools.permutations(range(n)):
  now = 0
  for i in range(n-1):
    if max(lr[res[i]][0],now) > lr[res[i+1]][1]:
      break
    else:
      now = max(lr[res[i]][0],now)
  else:
    ans += 1
    #print(res)
print(ans)