#include <bits/stdc++.h>

[[nodiscard]] static inline constexpr int_fast32_t solve(const uint_fast32_t K, const uint_fast32_t N, [[maybe_unused]] const uint_fast32_t F, const std::vector<uint_least32_t>& A) noexcept
{
	return std::max<int_fast32_t>(static_cast<int_fast32_t>(K * N) - std::accumulate(A.begin(), A.end(), 0), -1);
}

int main()
{
	std::cin.tie(nullptr);
	std::ios::sync_with_stdio(false);
	
	uint_fast32_t K, N, F;
	std::cin >> K >> N >> F;
	std::vector<uint_least32_t> A(F);
	for (auto& a : A) std::cin >> a;

	std::cout << solve(K, N, F, A) << '\n';
	return 0;
}