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#include <bits/stdc++.h>

using namespace std;

const int N = 32;

int h, w;
int n, d[N];

map<tuple<int, int, int>, long long> cache;

long long solve(int a, int b, int i) {
	// cerr << "solving: " << a << ' ' << b << ' ' << i << '\n';

	if (a == 0 || b == 0)
		return 0;

	assert(i >= 0);
	if (d[i] > a || d[i] > b)
		return solve(a, b, i - 1);
	
	auto key = tie(a, b, i);
	if (cache.count(key))
		return cache[key];

	int p = (a / d[i]) * d[i];
	int q = (b / d[i]) * d[i];

	if (p == a)
		return cache[key] = solve(a, b - q, i) + 1LL * (a / d[i]) * (b / d[i]);

	if (q == b)
		return cache[key] = solve(a - p, b, i) + 1LL * (a / d[i]) * (b / d[i]);

	return cache[key] = min(solve(a - p, b, i) + solve(p, b - q, i),
			  	 			solve(a - p, q, i) + solve(a, b - q, i)) 
							+ 1LL * (a / d[i]) * (b / d[i]);
}

int main() {
	ios::sync_with_stdio(false), cin.tie(nullptr);

	cin >> h >> w;
	cin >> n;
	for (int i = 0; i < n; i++)
		cin >> d[i];
	
	if (h % d[0] != 0 || w % d[0] != 0) {
		cout << "-1\n";
		return 0;
	}
	else {
		cout << solve(h, w, n - 1) << '\n';
	}
}