import java.util.Scanner;

public class No143 {
	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[] age = new int[f];
		int i = 0;

		for(int j = 0; j < age.length ; j++){
			age[j] = sc.nextInt();
			i += age[j];
		}
		System.out.println( (k * n >= i) ? k * n - i : -1 );
	}
}