import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int k = scan.nextInt();
        int n = scan.nextInt();
        int f = scan.nextInt();
        int[] a = new int[f];
        for (int i = 0; i < f; i++) {
            a[i] = scan.nextInt();
        }
        int ret = k * n;
        for (int i = 0; i < f; i++) {
            ret -= a[i];
            if (ret < 0) {
                ret = -1;
                break;
            }
        }
        System.out.println(ret);
        scan.close();
    }
}