fun main(args: Array<String>) {
    p361()
}

fun p361() {
    val (k, n, f) = readLine()!!.split(' ').map { it.toInt() }
    val aList = readLine()!!.split(' ').map { it.toInt() }

    val answer = (k * n - aList.sum()).let { if (it < 0) -1 else it }

    println(answer)
}