import java.util.Scanner;

class No_143 {

	public static void main(String args[]) {

		Scanner stdIn = new Scanner(System.in);		
		int beans = stdIn.nextInt();
		int pack = stdIn.nextInt();
		int family = stdIn.nextInt();

		//calicuLate(beans, pack, family);

		int sum = 0;
		for (int i = 0; i < family; i++) {
			sum += stdIn.nextInt();
		}
		if ( beans * pack < sum ) {
			System.out.println(-1);
		} else {
			System.out.println((beans * pack) - sum);
		}
	}
}