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#include<bits/stdc++.h>
using namespace std;
#ifdef LOCAL
template <class T, class U> auto &operator<<(ostream &out, pair<T, U> a) { return out << "(" << a.first << ", " << a.second << ")"; }
template <class T, class = class enable_if<!is_same<T, string>(), class T::iterator>::type> auto &operator<<(ostream &out, T a) {
    out << "{";
    bool fi = 1;
    for(auto b : a) {
        if(fi) {out<<b; fi=0;}
        else out<<", "<<b;
    }
    return out << "}";
}

template <class T, class X, class Y> auto &operator<<(ostream &out, const priority_queue<T,X,Y>& a) {auto b = a; vector<T> v; while(!b.empty()) {v.push_back(b.top()); b.pop();} return out<<v; }
void dump(){
   cerr << "\e[39m"<<"\n";
}
template <class T, class... Ts> void dump(T a, Ts... x) {
   cerr << a << ", ";
   dump(x...);
}
#define debug(...) cerr << "\e[91m"<<__func__<<":"<<__LINE__<<"\t"<<"[" #__VA_ARGS__ "]: ", dump(__VA_ARGS__)
#else
#define debug(...) ;
#endif

template<class C> C reversed(C c) {reverse(c.begin(),c.end()); return c;}
#define rep(i, n) for(int i=0;i<(int)(n);i++)
#define all(X) (X).begin(), (X).end()
#define mp make_pair
#define st first
#define nd second
typedef long long ll;
typedef pair<int,int> pii;


int32_t main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int h, w, n;
    cin >> h >> w >> n;

    vector<int> pictures(n);
    rep(i, n) cin >> pictures[i];

    ll res = 0;
    queue<pair<pair<int, int>, int>> q;
    q.push({{h, w}, n-1});

    while (!q.empty()) {
        auto [hw, i] = q.front();
        q.pop();

        if (i < 0) {
            cout << "-1\n";
            return 0;
        }

        h = hw.st;
        w = hw.nd;

        ll ct = (ll) (h / pictures[i]) * (w / pictures[i]);
        res += ct;

        int new_h = h % pictures[i];
        int new_w = w % pictures[i];

        if (new_h > 0) q.push({{new_h, w - new_w}, i-1});
        if (new_w > 0) q.push({{h, new_w}, i-1});
    }

    cout << res << "\n";

    return 0;
}