def nextl(i)
  if lucky(i)
    i.to_i
  else
    multiply = true
    (i.length - 1).downto(0) do |j|
      if i[j] < '4'
        i[j] = '4'
        multiply = false
      elsif i[j] >= '4' and i[j] < '7'
        i[j] = '7'
        multiply = false
        break
      elsif i[j] > '7'
        i[j] = '4'
      end
    end

    if multiply
      i = "4" * (i.length + 1)
    end

    i.to_i
  end
end