import scala.io.StdIn._
object Main extends App {

  val n = readInt
  val a = readLine.trim.split(' ').map(_.toLong).sorted
  val max = (for (i ← 0 until (n - 3)) yield {
    Rational(a(n - 1) + a(n - 2), a(i + 1) - a(i))
  }).max
  val res = List(makeByFour(a.takeRight(4)), max).max.let{case Rational(num, den) ⇒ num.toDouble / den}
  println(
    f"$res%15f"
  )

  def makeByFour(nums: Seq[Long]): Rational = {
    nums.permutations.map{a ⇒
      Rational(a(0) + a(1), math.abs(a(2) - a(3)))
    }.max
  }

  case class Rational(num: Long, den: Long) extends Ordered[Rational] {
    override def compare(that: Rational): Int = (num * that.den).compare(den * that.num)
  }

  implicit class Extension[T](val value: T) extends AnyVal {
    @inline def let[A](@inline func: T ⇒ A): A = func(value)
  }
}

