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#include <iostream>
#include <vector>
#include <algorithm>

struct obr
{
	uint32_t x;
	uint32_t y;
	std::vector<uint32_t> sizes;
};

void load(std::istream& in, obr& o)
{
	in >> o.x;
	in >> o.y;
	if (o.x > o.y)
	{
		auto tmp = o.y;
		o.y = o.x;
		o.x = tmp;
	}
	uint32_t num_squares = 0;
	in >> num_squares;
	o.sizes.reserve(num_squares);
	for (uint32_t i = 0; i < num_squares; i++)
	{
		uint32_t val;
		in >> val;
		o.sizes.insert(o.sizes.begin(), val);
	}
}

void print(std::ostream& out, obr& o)
{
	out << o.x << "x" << o.y << "\n";
	out << o.sizes.size() << "\n";
	uint32_t i = 0;
	for (auto it = o.sizes.begin(); it != o.sizes.end(); ++it)
	{
		out << "[" << i << "] : " << *it << "\n";
		i++;
	}
}

std::vector<uint32_t>::iterator find_first_lower(
		std::vector<uint32_t>::iterator &start,
		std::vector<uint32_t>::iterator &end,
		uint32_t lower
		)
{
	for (auto it = start; it != end; ++it)
	{
		if (*it <= lower)
		{
			return it;
		}
	}
	return end;
}

uint32_t count_stripe(std::vector<uint32_t> &sizes, uint32_t dim, 
	std::vector<uint32_t>::iterator &max_it)
{
	uint32_t result = 0;
	auto start = sizes.begin();
	auto end = sizes.end();
	auto it = find_first_lower(start, end, dim);
	if (it == sizes.end())
	{
		return 0;
	}
	else 
	{	auto factor = 1;
		auto size = *it;
		max_it = it;
		result += factor * (dim / size);
		auto next_dim = dim % size;
		while (next_dim != 0)
		{
			it = find_first_lower(it, end, next_dim);
			if (it == sizes.end()) {
				return 0;
			}
			else 
			{
				auto new_size = *it;
				factor *= size / new_size;
				size = new_size;
				result += factor * (next_dim / size);
				next_dim = next_dim % size;
			}
		}
	}
	return result;
}

bool verify_lowest_div(obr& o)
{
	auto lowest = o.sizes[o.sizes.size()-1];
	if ((o.x % lowest != 0) || (o.y % lowest) != 0)
	{
		return false;
	}
	return true;
}

void prog_main(std::istream& in, std::ostream& out)
{
	obr o;
	load(in, o);
	if (!verify_lowest_div(o))
	{
		out << "-1\n";
		return;	
	}
	std::vector<uint32_t>::iterator it;
	auto result = count_stripe(o.sizes, o.x, it);
	if (result == 0) 
	{
		out << "-1\n";
	}
	else 
	{	
		auto new_dim = o.y % *it;
		result *= (o.y / *it);
		while (new_dim > 0 && it != o.sizes.end()) 
		{
			o.sizes.erase(it);
			auto new_result = count_stripe(o.sizes, o.x, it);
			if (new_result == 0 || it == o.sizes.end())
			{
				out << "-1\n";
				return;
			}
			else 
			{
				auto newer_dim = new_dim % *it;
				new_result *= (new_dim / *it);
				result += new_result;
				new_dim = newer_dim;
			}
		}
		out << result << std::endl;
	}
}


#ifndef TEST
int main(int argc, char* argv[])
{
	prog_main(std::cin, std::cout);
	return 0;
}
#endif