import java.util.*;

public class Main {
	static Scanner sc = new Scanner(System.in);
	public static void main(String[] args) {
		int a = sc.nextInt();
		int b = sc.nextInt();
		int mame = a * b;
		int n = sc.nextInt();
		for (int i=0; i<n; i++) {
			int temp = sc.nextInt();
			mame -= temp;
		}
		System.out.println(mame<0?"-1":mame);
	}
}