import java.util.Scanner;

public class yukicoder_143 {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		
		String n = scanner.nextLine();
		String m = scanner.nextLine();
		
		
		String[] array = n.split(" ");
		int sum = Integer.parseInt(array[0]) * Integer.parseInt(array[1]);
		
		String[] array_fam = m.split(" ");
		
		int sum_fam = 0;
		
		for (int i = 0; i < array_fam.length; i++) {
			sum_fam += Integer.parseInt(array_fam[i]);
		}
		
		
		if (sum >= sum_fam) {
			System.out.println(sum - sum_fam);
		} else {
			System.out.println(-1);
		}
	}
}