W, H = gets.split.map(&:to_i)
M = (1..H).map {gets.split.map(&:to_i)}
dp = Array.new(H) {Array.new(W, false)}
def f(i, j, p, q, dp)
  if not i.between?(0, H - 1) or not j.between?(0, W - 1) or M[i][j] != M[p][q]
    false
  elsif dp[i][j]
    true
  else
    dp[i][j] = true
    [[i, j + 1], [i + 1, j], [i, j - 1], [i - 1, j]].any? {|a, b| [a, b] != [p, q] and f(a, b, i, j, dp)}
  end
end
ans = if (0...H).to_a.product((0...W).to_a).any? {|i, j| not dp[i][j] and f(i, j, i, j, dp)}
        "possible"
      else
        "impossible"
      end
puts ans