import java.util.Scanner;

class No_143 {

    public static void main(String args[]) {

        Scanner stdIn = new Scanner(System.in);
        int beans = stdIn.nextInt();
        int pack = stdIn.nextInt();
        int family = stdIn.nextInt();
        calculate(stdIn, beans, pack, family);
    }

    public static int calculate(Scanner stdIn, int beans, int pack, int family) {

        int sum = 0;
        int all = beans * pack;
        for (int i = 0; i < family; i++) {
            sum += stdIn.nextInt();
        }
        if (all > sum) {
            System.out.println(all - sum);
        } else {
            System.out.println(-1);
        }
        return 0;
    }
}