import java.util.Scanner

object mame {
  def main(args: Array[String]): Unit = {
    val sc = new Scanner(System.in)
    val k = sc.nextInt
    val n = sc.nextInt
    val f = sc.nextInt
    var lst = List[Int]()
    for (i <- 1 to f) {
      lst :+= sc.nextInt
    }

    var m = k*n - lst.sum
    if (m < 0){
      m = -1
    }
    println(m)
  }
}