import java.util.Scanner;

public class Main {

	static Scanner sc = new Scanner(System.in);

	public static void main(String[] args) {

		int k = sc.nextInt();
		int n = sc.nextInt();
		int mame = k * n;

		int f = sc.nextInt();

		int age = 0;
		for(int i = 0; i < f; i++) {
			age += sc.nextInt();
		}

		if(age > mame) {
			System.out.println("-1");
		} else {
			System.out.println(mame - age);
		}
    }
}