object Main extends App {

  import scala.io.StdIn._
  import scala.math._

  val n = readLine.trim.toInt
  val taboos = Array.tabulate(n){_ ⇒ readLine.trim}
  var left = 1
  var right = 10
  var result = ""
  while (left < right){
    val mid = (left + right) / 2
    search(mid, taboos) match {
      case Some(res) ⇒
        result = res
        right = mid
      case None ⇒
        left = mid + 1
    }
  }
  println(
    result
  )
  def search(length: Int, taboos: Array[String]): Option[String] = {
    var suspects = Nil: List[Long]
    for (str ← taboos.filter(_.length >= length)){
      for (last ← length to str.length){
        suspects ::= str.substring(last - length, last).foldLeft(0L){case (l, c) ⇒ l * 26 + (c - 'a')}
      }
    }
    firstNotFound(suspects.sorted.distinct) match {
      case i if exponent(26, length) > i ⇒
        var res: List[Char] = Nil
        var n  = i
        for (_ ← 0 until length) {
          res ::= ((n % 26) + 'a').toChar
          n /= 26
        }
        Some(res.mkString)
      case _ ⇒ None
    }
  }
  def exponent(base: Long, exp: Int): Long = {
    exp match {
      case 0 ⇒ 1L
      case 1 ⇒ base
      case _ ⇒ exponent(base, exp / 2).let{it ⇒ it * it * exponent(base, exp % 2)}
    }
  }
  def firstNotFound(list:List[Long], current: Long = 0L): Long ={
    list match {
      case h::t if current == h ⇒ firstNotFound(t, current + 1)
      case _ ⇒ current
    }
  }
  implicit class Extension[T](val value :T){
    def also(func: T ⇒ Unit): T = {
      func(value)
      value
    }
    def let[R](func: T ⇒ R): R = func(value)
  }
}
