h, w = gets.not_nil!.split(" ").map{|x| x.to_i}
rect = [] of Array(String)
1.upto(h) do |i|
  rect << gets.not_nil!.split("")
end

def ok?(rect, h, w)
  (0...h).to_a.product((0...w).to_a) do |dy, dx|
    next if dx == 0 && dy == 0
    _rect = rect.clone
    ok = true

    (0...h).to_a.product((0...w).to_a) do |y, x|
      next if _rect[y][x] == "."
      ny = y + dy
      nx = x + dx

      if nx >= w || ny >= h
        ok = false
        break
      end

      if _rect[y][x] == _rect[ny][nx]
        _rect[y][x] = "."
        _rect[ny][nx] = "."
      else
        ok = false
        break
      end
    end

    return true if ok
  end
  return false
end

puts ok?(rect, h, w) ? "YES" : "NO"