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#include <iostream>
using namespace std;

int obr[30], n;

long long ile(int h, int w)
{
	int act, rh, rw;
	long long ret, x;
	ret = 0; act = 0;
	while (act < n && (obr[act] > h || obr[act] > w))
		act++;
	if (act == n) return -1;
	ret = (h / obr[act]) * (long long)(w / obr[act]);
	rh = h % obr[act]; rw = w % obr[act];
	if (rh == 0 && rw == 0) return ret;
	if (rh != 0)
	{
		x = ile(rh, w - rw);
		if (x == -1) return -1;
		ret += x;
	}
	if (rw != 0)
	{
		x = ile(h - rh, rw);
		if (x == -1) return -1;
		ret += x;
	}
	if (rh != 0 && rw != 0)
	{
		x = ile(rh, rw);
		if (x == -1) return -1;
		ret += x;
	}
	return ret;
}

int main() 
{
	int h, w;
	int i;
	cin >> h >> w >> n;
	for (i = n - 1; i >= 0; i--) cin >> obr[i];
	
	cout << ile(h, w);
	
	return 0;
}