import kotlin.math.max

fun main() {
    val builder = StringBuilder()

    val (k, n, _) = readInputLine().split(" ").map { it.toInt() }

    var rem = k * n

    readInputLine().split(" ").forEach { rem -= it.toInt() }

    builder.appendln(max(rem, -1))

    print(builder.toString())
}

fun readInputLine(): String {
    return readLine()!!
}