import java.util.Scanner;

public class Answer {
	public static void main(String str[]) {
        Scanner sc = new Scanner(System.in);

        int beanNum = Integer.valueOf(sc.next());
        int hukuroNum = Integer.valueOf(sc.next());
        int familyNum = Integer.valueOf(sc.next());

        int rest = beanNum * hukuroNum;
        for(int i=0; i<familyNum; i++) {
        	rest -= Integer.valueOf(sc.next());
        }
        if(rest < 0) {
        	System.out.println("-1");
        } else {
        	System.out.println(rest);
        }
	}
}