output_description: If we can visit all the towns by traversing each of the roads exactly once,
  print `YES`; otherwise, print `NO`.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      a_1 b_1
      a_2 b_2
      a_3 b_3
attempt: from collections import defaultdict, Counter
  from itertools import product, groupby, count, permutations, combinations
  from math import pi, sqrt
  from collections import deque
  from bisect import bisect, bisect_left, bisect_right
  from string import ascii_lowercase
  from functools import lru_cache
  import sys
  
  sys.setrecursionlimit(10000)
  INF = float("inf")
  YES, Yes, yes, NO, No, no = "YES", "Yes", "yes", "NO", "No", "no"
  dy4, dx4 = [0, 1, 0, -1], [1, 0, -1, 0]
  dy8, dx8 = [0, -1, 0, 1, 1, -1, -1, 1], [1, 0, -1, 0, 1, 1, -1, -1]
  
  
  def inside(y, x, H, W):
      return 0 <= y < H and 0 <= x < W
  
  
  def ceil(a, b):
      return (a + b - 1) // b
  
  
  # aとbの最大公約数
  def gcd(a, b):
      if b == 0:
          return a
      return gcd(b, a % b)
  
  
  # aとbの最小公倍数
  def lcm(a, b):
      g = gcd(a, b)
      return a / g * b
  
  
  def main():
      g = defaultdict(list)
      for _ in range(3):
          a, b = map(int, input().split())
          g[a].append(b)
          g[b].append(a)
  
      for p in permutations([1, 2, 3, 4]):
          ok = True
          for i in range(3):
              a, b = p[i], p[i + 1]
              if b not in g[a]:
                  ok = False
          if ok:
              print(YES)
              return
      print(NO)
  
  
  if __name__ == "__main__":
      main()
  
problem_description: Statement
  
  There are four towns, numbered 1,2,3 and 4. Also, there are three roads. The
  i-th road connects different towns a_i and b_i bidirectionally. No two roads
  connect the same pair of towns. Other than these roads, there is no way to
  travel between these towns, but any town can be reached from any other town
  using these roads.
  
  Determine if we can visit all the towns by traversing each of the roads
  exactly once.
samples: [{"input": "4 2\n    1 3\n    2 3", "output": "YES\n    \n\nWe can visit all the towns in the order 1,3,2,4.\n\n* * *"}, {"input": "3 2\n    2 4\n    1 2", "output": "NO\n    \n\n* * *"}, {"input": "2 1\n    3 2\n    4 3", "output": "YES"}]