struct DSU
  property p : Array(Int32)
  
  def initialize(n : Int32)
    @p = Array.new(n, -1)
  end
  
  def root(x : Int32) : Int32
    @p[x] < 0 ? x : (@p[x] = root(@p[x]))
  end
  
  def unite(x : Int32, y : Int32)
    x = root(x)
    y = root(y)
    return if x == y
    
    if @p[x] > @p[y]
      x, y = y, x
    end
    @p[x] += @p[y]
    @p[y] = x
  end
  
  def same(x : Int32, y : Int32) : Bool
    root(x) == root(y)
  end
end

dx = [0, 1]
dy = [1, 0]

input = gets.not_nil!.split
w = input[0].to_i
h = input[1].to_i

m = Array.new(h) { gets.not_nil!.split.map(&.to_i) }

uf = DSU.new(h * w)

# Process vertical connections
(0...h - 1).each do |i|
  (0...w).each do |j|
    if m[i][j] == m[i + 1][j]
      uf.unite(i * w + j, (i + 1) * w + j)
    end
  end
end

# Process horizontal connections and check for cycles
(0...h).each do |i|
  (0...w - 1).each do |j|
    next unless m[i][j] == m[i][j + 1]
    
    if uf.same(i * w + j, i * w + j + 1)
      puts "possible"
      exit
    end
    uf.unite(i * w + j, i * w + j + 1)
  end
end

puts "impossible"