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#include <bits/stdc++.h>
using namespace std;
using lint = long long;
using pi = array<lint, 2>;
#define sz(v) ((int)(v).size())
#define all(v) (v).begin(), (v).end()
#define cr(v, n) (v).clear(), (v).resize(n);

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	int h, w, n;
	cin >> h >> w >> n;
	vector<int> a(n);
	for (auto &x : a)
		cin >> x;
	if (h % a[0] || w % a[0]) {
		cout << "-1\n";
		return 0;
	}
	h /= a[0];
	w /= a[0];
	lint ans = 0;
	for (int i = 0; i < n - 1; i++) {
		int d = a[i + 1] / a[i];
		ans += 1ll * h * w - 1ll * (h - h % d) * (w - w % d);
		h /= d;
		w /= d;
	}
	ans += 1ll * h * w;
	cout << ans << "\n";
}