import java.util.Scanner;

public class Main {

	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[] a = new int[f];
		int total = 0;
		for(int i = 0 ; i < f ; i++) {
			a[i] = sc.nextInt();
			total += a[i];
		}
		if(k * n >= total) {
			System.out.println(k * n - total);
		} else {
			System.out.println(-1);
		}
 	}
}