import java.util.Scanner;

public class Mame {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int K = s.nextInt(), N = s.nextInt(), F = s.nextInt(), sum = 0;
		for(int i = 0; i < F;i++){
			sum += s.nextInt();
		}
		
		if(sum > N*K){
			System.out.println(-1);
		}else{
			System.out.println(N*K - sum);
		}
	}

}