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();

		calicuLate(stdIn, beans, pack, family);

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

	public static int calicuLate(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;
	}
}