import java.util.Scanner

import scala.collection.mutable

object Problem247 {

  case class State(count: Int, total: Int)

  def proc(total: Int, n: Int, ai: Seq[Int]): Int = {
    if (total % ai.max == 0) {
      return total / ai.max
    }

    val dp = Array.fill(total + 1)(Int.MaxValue)

    val q = new mutable.PriorityQueue[State]()(Ordering.by(_.count))
    q += State(0, 0)

    while (!q.isEmpty) {
      val s = q.dequeue()

      for (a <- ai) {
        if (s.total + a <= total) {
          val next = State(s.count + 1, s.total + a)
          if (next.count < dp(next.total)) {
            q += next
            dp(next.total) = next.count
          }
        }
      }
    }

    dp(total) match {
      case Int.MaxValue => -1
      case x => x
    }
  }

  def main(args: Array[String]) {
    val sc = new Scanner(System.in)
    val total = sc.nextInt()
    val n = sc.nextInt()
    val ai = Seq.fill(n)(sc.nextInt())
    val result = proc(total, n, ai)
    println(result)
  }
}