#include <iostream>

constexpr int max_n = 40;
long long input[max_n];

int main()
{
	std::ios_base::sync_with_stdio(0);
	std::cin.tie(0);

	int h, w;
	std::cin >> h >> w;

	int n;
	std::cin >> n;

	for (int i = 0; i < n; ++i)
		std::cin >> input[i];
	
	long long output = 0ll;
	int j = n - 1;
	while (h > 0 && j >= 0)
	{
		int count = h / input[j];
		h %= input[j];
		long long cost = 0ll;

		int w2 = w;
		int i = j;
		while (w2 > 0 && i >= 0)
		{
			cost += (w2 / input[i]) * (input[j] / input[i]);
			w2 %= input[i];
			--i;
		}

		if (w2 != 0)
		{
			std::cout << "-1\n";
			return 0;
		}

		output += cost * count;
		--j;
	}
	if (h != 0)
	{
		std::cout << "-1\n";
		return 0;
	}
	std::cout << output << '\n';
}