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#pragma GCC optimize ("O3")
#include <bits/stdc++.h>
using namespace std;

#define FOR(i, b, e) for(int i = (b); i < (e); i++)
#define sz(x) int(x.size())
#define all(x) x.begin(), x.end()
#define pb push_back
#define mp make_pair
#define st first
#define nd second
using ll = long long;
using vi = vector<int>;
using pii = pair<int, int>;

auto &operator<<(auto &o, pair<auto, auto> p) {
	return o << "(" << p.st << ", " << p.nd << ")"; }
auto operator<<(auto &o, auto x)->decltype(end(x), o) {
	o << "{"; int i=0; for(auto e: x) o << ", " + 2*!i++ << e;
	return o << "}"; }
#ifdef LOCAL
#define deb(x...) cerr << "[" #x "]: ", [](auto...$) { \
					((cerr << $ << "; "),...) << endl; }(x)
#else
#define deb(...)
#endif

const ll INF = 2e18;

vi arr;

ll licz(int h, int w, int ind) {
	if(1ll * h * w == 0) return 0;
	if(ind == -1) return INF;
	int rh = h / arr[ind], rw = w / arr[ind];
	ll obrazy = 1ll * rh * rw;
	rh = h % arr[ind];
	rw = w % arr[ind];
	obrazy += licz(rh, w - rw, ind - 1) + licz(h, rw, ind - 1);
	return min(obrazy, INF);
}

void solve() {
	int h, w, n;
	cin >> h >> w >> n;
	arr = vi(n);
	for(int &x: arr) cin >> x;
	ll ans = licz(h, w, n - 1);
	if(ans == INF) ans = -1;
	cout << ans << '\n';
}

int main() {
	cin.tie(0)->sync_with_stdio(0);
	int tt = 1;
	// cin >> tt;
	FOR(te, 0, tt) solve();
	return 0;
}