import java.util.Scanner;
public class Main{
	public static void main(String[] args) {
		//System.out.println("袋に入っている豆の数");
		Scanner scanner = new Scanner(System.in);
		int K = scanner.nextInt();
		//System.out.println("袋の数");
		int N = scanner.nextInt();

		//System.out.println("家族の人数");
		int F = scanner.nextInt();
		long b = K*N;	//豆の数
		int []A = new int[F];	//家族の年齢
		long e = 0;	//必要な豆の数
		for(int i = 0; i < F; i++) {
			A[i] = scanner.nextInt();
			e += A[i];
		}
		scanner.close();
		if(e <= b) {
			System.out.println(b - e);
		}else {
			System.out.println(-1);
		}
	}
}