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

#define fwd(i, a, n) for (int i = (a); i < (n); i ++)
#define rep(i, n) fwd(i, 0, n)
#define all(X) begin(X), end(X)
#define sz(X) ((int)X.size())
#define st first
#define nd second
#define pii pair<int, int>
#define vi vector<int>
#define ll long long

#ifdef LOC
auto &operator << (auto &out, pair<auto, auto> a) {
	return out << "(" << a.st << ", " << a.nd << ")";
}

auto &operator << (auto &out, auto a) {
	out << "{";
	for (auto b : a)
		out << b << ", ";
	return out << "}";
}

void dump(auto... x) { ((cerr << x << ", "), ...) << '\n'; }

#define debug(x...) cerr << "[" #x "]: ", dump(x)
#else
#define debug(...) 0
#endif

vector<ll> a;

ll magic(ll h, ll w) {
	if (h * w == 0)
		return 0;
	ll big = 0;
	for (auto x : a)
		if (x <= min(h, w))
			big = x;
	ll cnt = (h / big) * (w / big);
	return cnt + 
	magic(h % big, w) + 
	magic(h - h % big, w % big);
}

int32_t main() {
   	ios_base::sync_with_stdio(0), cin.tie(0);
	ll h, w;
	cin >> h >> w;
	int n;
	cin >> n;
	a.resize(n);
	rep(i, n)
		cin >> a[i];
	if (h % a[0] != 0 || w % a[0] != 0) {
		cout << "-1\n";
		return 0;
	}
	cout << magic(h, w) << '\n';
   	return 0;
}