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#include <bits/stdc++.h>
using namespace std;

int64_t Count(int64_t h, int64_t w, int64_t dh, int64_t dw)
{
	return h * w - (h - dh) * (w - dw);
}

int main()
{
	int h, w, n;
	vector<int> d;
	scanf("%d%d%d", &h, &w, &n);
	d.resize(n);
	for (int i = 0; i < n; i++)
		scanf("%d", &d[i]);
	int64_t count = 0;
	if (h % d[0] != 0 || w % d[0] != 0)
	{
		printf("-1\n");
		return 0;
	}
	for (int i = 0; i < n - 1; i++)
	{
		int rh = h % d[i + 1];
		int rw = w % d[i + 1];
		count += Count(h / d[i], w / d[i], rh / d[i], rw / d[i]);
		h -= rh;
		w -= rw;
	}
	count += int64_t(h / d.back()) * (w / d.back());
	printf("%ld\n", count);
	return 0;
}