import java.util.Scanner;

public class No143 {

    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        int beans = s.nextInt();
        int packages = s.nextInt();
        int people = s.nextInt();
        int sum = 0;
        for (int i = 0; i < people; i++) {
            sum += s.nextInt();
        }
        if (sum > beans * packages) {
            System.out.println(-1);
        } else {
            System.out.println(beans * packages - sum);
        }
    }
}