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: print("YNEOS"[sum(map(int, open(0).read().split())) < 15 :: 2])
  
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"}]