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#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

vector<int> obrazy;

pair<long long, pair<int, int>> f(int h, int w) {
    if (h > w)
        swap(h, w);

    int hleft = h, builtw = 0;
    long long ile = 0;
    for (auto it : obrazy) {
        if (it <= hleft) {
            if (builtw == 0) {
                builtw = it;
            }
            long long countpion = hleft / it;
            ile += countpion * builtw / it;
            hleft %= it;
        }
    }
    ile *= w / builtw;
    w %= builtw;
    return {ile, {h, w}};
}

int main() {
    int h, w, n, x;
    cin >> h >> w;
    cin >> n;
    for (int i = 1; i <= n; i++) {
        cin >> x;
        obrazy.push_back(x);
    }
    reverse(obrazy.begin(), obrazy.end());

    if (h % obrazy.back() != 0 || w % obrazy.back() != 0) {
        cout << -1;
        return 0;
    }

    long long wyn = 0;

    while (h > 0 && w > 0) {
        auto ret = f(h, w);
        wyn += ret.first;
        h = ret.second.first;
        w = ret.second.second;
    }

    cout << wyn;
    return 0;
}