import java.util.Scanner;
public class Yukicoder {

	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 sum = 0;
		int result = 0;

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

		if (K * N - sum < 0){
			result = -1;
		}else{
			result = K * N -sum;
		}

		System.out.println(result);

	}

}