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#include <bits/stdc++.h>

using namespace std;

#define rep(i, a, b) for (int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<ll> vl;

#ifdef LOCAL
ostream &operator<<(ostream &out, string str)
{
    for (char c : str)
        out << c;
    return out;
}
template <class L, class R>
ostream &operator<<(ostream &out, pair<L, R> p) { return out << "(" << p.first << ", " << p.second << ")"; }
template <class L, class R, class S>
ostream &operator<<(ostream &out, tuple<L, R, S> p)
{
    auto &[a, b, c] = p;
    return out << "(" << a << ", " << b << ", " << c << ")";
}
template <class T>
auto operator<<(ostream &out, T a) -> decltype(a.begin(), out)
{
    out << '{';
    for (auto it = a.begin(); it != a.end(); it = next(it))
        out << (it != a.begin() ? ", " : "") << *it;
    return out << '}';
}
void dump() { cerr << "\n"; }
template <class T, class... Ts>
void dump(T a, Ts... x)
{
    cerr << a << ", ";
    dump(x...);
}
#define debug(...) cerr << "[" #__VA_ARGS__ "]: ", dump(__VA_ARGS__)
#else
#define debug(...) 42
#endif

main()
{
    cin.tie(0)->sync_with_stdio(0);
    cin.exceptions(cin.failbit);

    int Z = 1;
    // cin >> Z;
    while (Z--)
    {
        ll h, w;
        cin >> h >> w;
        int n;
        cin >> n;
        vl V(n);
        for (auto &a : V)
            cin >> a;
        if (h % V[0] || w % V[0])
        {
            cout << -1 << endl;
            continue;
        }
        ll sum = 0;
        int it = 1;
        while (it < n)
        {
            sum += (h * w - (h / V[it]) * V[it] * (w / V[it]) * V[it]) / (V[it - 1] * V[it - 1]);
            h = (h / V[it]) * V[it];
            w = (w / V[it]) * V[it];
            it++;
        }
        sum += (h * w) / (V.back() * V.back());
        cout << sum << endl;
    }
}