import java.util.Scanner;

public class Main {

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

		int total = K * N;

		for(int i = 0; i < F; i++){
			total -= scan.nextInt();
		}

		if(total < 0){
			System.out.println(-1);
		}else{
			System.out.println(total);
		}
		scan.close();
	}
}