output_description: If it is impossible to divide the vertices into sets so that the condition is
  satisfied, print -1. Otherwise, print the maximum possible number of sets, k,
  in a division that satisfies the condition.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
      S_{1,1}...S_{1,N}
      :
      S_{N,1}...S_{N,N}
attempt: N = int(input())
  S = [input() for i in range(N)]
  l = [None] * N
  l[0] = 0
  a = 0
  while None in l:
      for i in range(1, N):
          for j in range(N):
              if S[i][j] == "1":
                  if l[i] == None:
                      if l[j] == 1:
                          l[i] = 0
                      elif l[j] == 0:
                          l[i] = 1
                  elif l[i] == l[j]:
                      a = -1
                      break
  print(a)
  
problem_description: Statement
  
  Given is a connected undirected graph with N vertices and M edges. The
  vertices are numbered 1 to N, and the edges are described by a grid of
  characters S. If S_{i,j} is `1`, there is an edge connecting Vertex i and j;
  otherwise, there is no such edge.
  
  Determine whether it is possible to divide the vertices into non-empty sets
  V_1, \dots, V_k such that the following condition is satisfied. If the answer
  is yes, find the maximum possible number of sets, k, in such a division.
  
    * Every edge connects two vertices belonging to two "adjacent" sets. More formally, for every edge (i,j), there exists 1\leq t\leq k-1 such that i\in V_t,j\in V_{t+1} or i\in V_{t+1},j\in V_t holds.
samples: [{"input": "2\n    01\n    10", "output": "2\n    \n\nWe can put Vertex 1 in V_1 and Vertex 2 in V_2.\n\n* * *"}, {"input": "3\n    011\n    101\n    110", "output": "-1\n    \n\n* * *"}, {"input": "6\n    010110\n    101001\n    010100\n    101000\n    100000\n    010000", "output": "4"}]