#include <iostream>

int main() {
	int k, n, f;
	std::cin >> k >> n >> f;
	int amount = k * n;
	for (; f > 0; --f) {
		int a;
		std::cin >> a;
		amount -= a;
	}
	if (amount >= 0) {
		std::cout << amount;
	}
	else {
		std::cout << -1;
	}
	std::cout << std::endl;
	return 0;
}