fun main(){
    val knf = readLine()!!.split(" ").map(String::toInt)
	val a = readLine()!!.split(" ").map(String::toInt)    
    
    val ans = (knf[0] * knf[1]) - a.sum()
    if(ans >= 0){
        println(ans)
    }else{
        println("-1")
    }
}