import java.util.Scanner;

public class No143 {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int K = sc.nextInt();
		int N = sc.nextInt();
		int F = sc.nextInt();

		int ageSum = 0;
		for (int i = 1; i <= F; i++) {
			int age = sc.nextInt();
			ageSum += age;
		}

		if (K * N >= ageSum) {
			System.out.println(K * N - ageSum);
		} else {
			System.out.println(-1);
		}
	}

}