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/*
 * Author: toko
 * Time: 2024-03-14 19:46:50
 */

#ifndef DEBUG
#define NDEBUG
#pragma GCC optimize("Ofast,unroll-loops")
#else
#pragma GCC optimize("Og")
#endif

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main()
{
	iostream::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);

	long long h, w, n;
	cin >> h >> w >> n;
	vector<long long> d(n);
	for (int i = 0; i < n; i++)
		cin >> d[i];

	sort(d.begin(), d.end());

	if (h % d[0] != 0 || w % d[0] != 0)
	{
		cout << -1 << endl;
		return 0;
	}

	long long cnt = 0;

	for (int i = 1; i < n; i++)
	{
		long long n_w = w / d[i] * d[i];
		long long n_h = h / d[i] * d[i];
		cnt += (w * h - n_w * n_h) / d[i - 1] / d[i - 1];
		w = n_w;
		h = n_h;
	}

	cnt += (w * h) / d[n - 1] / d[n - 1];

	cout << cnt << endl;

	return 0;
}