import java.util.Scanner;

public class Run {
    public static void main (String[] arg) {
        Scanner scan = new Scanner(System.in);
        int K = scan.nextInt();
        int N = scan.nextInt();
        int F = scan.nextInt();
        int[] A = new int[F];
        if (K < 1||K > 1000||N < 1||N > 1000||F < 1||F > 100) System.exit(1);
        int sum = 0;
        for (int i = 0; i < F; i++) {
            A[i] = scan.nextInt();
            if (A[i] < 1||A[i] > 100) System.exit(1);
            sum += A[i];
        }

        int left = K * N - sum;
        if (left < 0) System.out.println(-1);
        else System.out.println(left);
    }
}