output_description: If there is no induced subgraph of G that satisfies the condition, print `-1`.
  Otherwise, print an induced subgraph of G that satisfies the condition, in the
  following format:
  
      
      
      K
      v_1
      v_2
      :
      v_K
      
  
  This represents the induced subgraph of G with K vertices whose vertex set is
  \\{v_1, v_2, \ldots, v_K\\}. (The order of v_1, v_2, \ldots, v_K does not
  matter.) If there are multiple subgraphs of G that satisfy the condition,
  printing any of them is accepted.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N M
      A_1 B_1
      A_2 B_2
      :
      A_M B_M
attempt: print("3\n1\n2\n4\n")
  
problem_description: Statement
  
  Given is a directed graph G with N vertices and M edges.  
  The vertices are numbered 1 to N, and the i-th edge is directed from Vertex
  A_i to Vertex B_i.  
  It is guaranteed that the graph contains no self-loops or multiple edges.
  
  Determine whether there exists an induced subgraph (see Notes) of G such that
  the in-degree and out-degree of every vertex are both 1. If the answer is yes,
  show one such subgraph.  
  Here the null graph is not considered as a subgraph.
samples: [{"input": "4 5\n    1 2\n    2 3\n    2 4\n    4 1\n    4 3", "output": "3\n    1\n    2\n    4\n    \n\nThe induced subgraph of G whose vertex set is \\\\{1, 2, 4\\\\} has the edge set\n\\\\{(1, 2), (2, 4), (4, 1)\\\\}. The in-degree and out-degree of every vertex in\nthis graph are both 1.\n\n* * *"}, {"input": "4 5\n    1 2\n    2 3\n    2 4\n    1 4\n    4 3", "output": "-1\n    \n\nThere is no induced subgraph of G that satisfies the condition.\n\n* * *"}, {"input": "6 9\n    1 2\n    2 3\n    3 4\n    4 5\n    5 6\n    5 1\n    5 2\n    6 1\n    6 2", "output": "4\n    2\n    3\n    4\n    5"}]