import java.util.Scanner; public class Main { public static void main(String[] args) { // 標準入力から読み込む際に、Scannerオブジェクトを使う。 Scanner sc = new Scanner(System.in); int k = sc.nextInt(); int n = sc.nextInt(); int f = sc.nextInt(); int total = k * n; int eat = 0; int remaining = 0; for(int i = 0; i < f; i++) { int a = sc.nextInt(); total -= a; } if(total <= 0) { System.out.println(-1); }else { System.out.println(total); } } }