package Yukicoder

/**
 * Created by hichikawa on 2015/11/02.
 */
fun main(args: Array<String>) {
    fun readLineIntArray() : List<Int> {
        val str = readLine() as String
        val arrStr = str.split(" ")
        val ret = arrStr.map { it.toInt() }
        return ret
    }
    val inputBeans = readLineIntArray()
    var numBeans = inputBeans[0] * inputBeans[1]
    val personsFimillyAge = readLineIntArray()
    for (num in personsFimillyAge) {
        numBeans -= num
        if (numBeans < 0) {
            println(-1)
            return
        }
    }
    println(numBeans)
}