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#include <iostream>

using namespace std;

typedef long long int lint;

int n;
const int MAX_DS = 30;
lint d[MAX_DS];

lint solve(lint h, lint w, int max_index) {
    // for (int i = 0; i < n - max_index - 1; ++i) {
    //     cout << "  ";
    // }
    // cout << "solve(h=" << h << ", w=" << w << ", i=" << max_index << ") = ";

    lint curr_d = d[max_index];

    if (h == 0 || w == 0) {
        //cout << "0 [end - no space]" << endl;
        return 0;
    }
    lint kh = h / curr_d;
    lint kw = w / curr_d;
    if (kh == 0 || kw == 0) {
        //cout << "deeper" << endl;
        return solve(h, w, max_index-1);
    }
    if (max_index == 0) {
        //cout << kh * kw << " [end - smallest image]" << endl;
        return kh * kw;
    }

    //cout << kh * kw << " + deeper" << endl;
    return kh * kw + solve(h - kh * curr_d, kw * curr_d, max_index - 1) + 
        solve(h, w - kw*curr_d, max_index - 1);
}

int main() {
    ios_base::sync_with_stdio(0);
    int h, w; cin >> h >> w;
    cin >> n;
    for (int i = 0; i < n; ++i)
        cin >> d[i];
    
    if (h % d[0] != 0 || w % d[0] != 0) {
        cout << "-1" << endl;
        return 0;
    }

    cout << solve(h, w, n-1) << endl; 
    return 0;
}