import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        short k = sc.nextShort();
        short n = sc.nextShort();
        short f = sc.nextShort();

        int bean = k * n;

        for (int i = 0; i < f; i++) {
            bean -= sc.nextShort();
        }

        if (bean < 0) {
            System.out.println(-1);
        } else {
            System.out.println(bean);
        }

    }
}