import java.util.Scanner
object no143{
    def main(args:Array[String]):Unit = {
        val scan = new Scanner(System.in)
        val n,k,f = scan.nextInt
        val a = for(i <- 1 to f) yield scan.nextInt
        if(a.sum <= n*k) println(n*k-a.sum) else println(-1)
    }
}