import java.util.*;

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