import java.util.Scanner;

public class A000143 {
	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 count = 0;
		for(int i=0;i<f;i++) {
			count += sc.nextInt();
		}
		int r = (k*n-count)>-1?k*n-count:-1;
		sc.close();

		System.out.println(r);
	}
}