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#include <bits/stdc++.h>

#define int long long

using namespace std;

int go(int w, int h, vector<int> &a) {
    if (!w || !h) return 0;
    if (w > h) return go(h, w, a);
    if ((w % a.back()) || (h % a.back())) return -1;
    int oldw = w;
    int res = 0;
    int curh = 0;
    for (int v : a) {
        if (v > w) continue;
        if (!curh) curh = v;
        int rem = w / v;
        w %= v;
        // cerr << "using " << v << " w is now " << w << endl;
        res += (curh / v) * rem;
    }
    if (w) return -1;
    int covh = h / curh;
    h %= curh;
    res *= covh;
    int rest = go(oldw, h, a);
    if (res == -1) return -1;
    return rest + res;
}

void solve() {
    int w, h;
    cin >> w >> h;
    int n;
    cin >> n;
    vector<int> a(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    reverse(a.begin(), a.end());
    cout << go(w, h, a) << endl;
}

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);

// #ifdef LOCAL
//     int t; cin >> t; while (t--)
// #endif

    solve();

    cout.flush();
}