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#include <bits/stdc++.h>

using namespace std;
using LL = long long;
#define FOR(i, l, r) for(int i = (l); i <= (r); ++i)
#define REP(i, n) FOR(i, 0, (n) - 1)
#define ssize(x) int(x.size())

template<class A, class B> auto &operator<<(ostream &o, pair<A, B> p) {
    return o << '(' << p.first << ", " << p.second << ')';
}

template<class T> auto operator<<(ostream &o, T x) -> decltype(x.end(), o) {
    o << '{'; int i = 0; for (auto e : x) o << (", ")+2*!i++ << e;
    return o << '}';
}

#ifdef DEBUG
#define debug(x...) cerr << "[" #x "]: ", [](auto... $) {((cerr << $ << "; "), ...); }(x), cerr << '\n';
#else
#define debug(...) {}
#endif


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

    LL h, w;
    int n;
    cin >> h >> w >> n;
    vector<LL> d(n);
    REP(i, n)
        cin >> d[i];
    reverse(d.begin(), d.end());
    bool lost = false;

    function<LL(LL, LL)> solve = [&](LL x, LL y) {
        if (x > y)
            swap(x, y);
        if (x == 0)
            return 0LL;

        for (auto &it : d) {
            if (it > x)
                continue;
            LL a = x / it;
            LL b = y / it;
            LL left_a = x - a * it;
            LL left_b = y - b * it;
            return a * b + solve(left_a, y) + solve(left_b, x - left_a);
        }

        lost = true;
        return 0LL;
    };

    LL res = solve(h, w);
    if (lost)
        cout << "-1\n";
    else
        cout << res << '\n';

    return 0;
}