import java.util.Scanner;

class Main{
	public static void main(String args[]){
		Scanner sc = new Scanner(System.in);

		int K = Integer.parseInt(sc.next());
		int N = Integer.parseInt(sc.next());
		int F = Integer.parseInt(sc.next());

		int sum = 0;

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

		if(K * N < sum){
			System.out.println(-1);
		}
		else{
			System.out.println( K * N - sum );
		}
	}
}