import java.util.Scanner;

public class Main {
	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);
		int k = sc.nextInt();// k粒
		int n = sc.nextInt();// n袋
		int f = sc.nextInt();// f人家族

		int sum = k * n;

		for (int i = 0; i < f; i++) {
			int a = sc.nextInt();// 年齢
			sum -= a;
		}
		if (sum >= 0) {
			System.out.println(sum);
		} else {
			System.out.println(-1);
		}
	}
}