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#include <bits/stdc++.h>
using namespace std;
 
#ifdef DEBUG
auto &operator<<(auto &o, pair<auto, auto> p) {
    return o << "()" << p.first << ", " << p.second <<")";
}
auto operator<<(auto &o, auto x)-> decltype(x.end(), o) {
    o << "{";int i = 0;
    for(auto e : x) o << ", "+!i++<<e;
    return o <<"}";
}
#define debug(x...) cerr << "["#x"]: ",[](auto...$){((cerr<<$<<"; "),...)<<endl;}(x)
#else
#define debug(...) {}
#endif
 

#define all(x) x.begin(), x.end()
#define pb push_back
#define fi first
#define se second
typedef pair <int, int> pii;

constexpr int INF = 1e9+1; 
map <pii, long long> M;
bool fail;

long long rek(int h, int w, int p, vector <int> & x) {
    if (h == 0 || w == 0 || p == -1) {
        if (h == 0 || w == 0) return 0;
        else return fail = true;
    }
    debug(h, w);
    if (h < w) swap(h, w);
    if (M.find({h, w}) == M.end()) {
        long long a = h/x[p];
        long long b = w/x[p];

        M[{h, w}] = a*b 
                    + rek(h % x[p], w, p-1, x)
                    + rek(h - (h % x[p]), w % x[p], p-1, x);
    }
    return M[{h, w}];
}

void test() {
    int h, w, n; cin>>h>>w>>n;
    vector <int> v(n);

    for (int i=0; i<n; ++i) {
        cin>>v[i];
    }

    long long x = rek(h, w, v.size()-1, v);
    if (fail) {
        cout<<-1<<'\n';
    }
    else {
        cout<<x<<'\n';
    }
}
 
signed main() {
  ios_base::sync_with_stdio(0); cin.tie(0);
  int t = 1;
  while (t--) {
    test();
  }
}