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#include <bits/stdc++.h>

long long h, w;
long long n;
std::vector<long long> T;

long long find_top_score(long long a, long long b) {
    long long output = 0;
    //std::cout << a << " " << b << std::endl;
    int idx = 0;
    while (T[idx] > a || T[idx] > b) {
        idx++;
    }
    output += (a / T[idx]) * (b / T[idx]);
    long long new_a = a - (a / T[idx]) * T[idx];
    long long new_b = b - (b / T[idx]) * T[idx];
    if (new_a != 0) {
        output += find_top_score(new_a, b - new_b);
    }
    if (new_b != 0) {
        output += find_top_score(a - new_a, new_b);
    }
    if (new_a != 0 && new_b != 0) {
        output += find_top_score(new_a, new_b);
    }
    return output;
}

int main() {
    // std::ios_base::sync_with_stdio(false);
    // std::cin.tie(NULL);
    // std::cout.tie(NULL);

    std::cin >> h >> w;
    std::cin >> n;
    for (int i = 0; i < n; i++) {
        long long tmp;
        std::cin >> tmp;
        T.push_back(tmp); 
    }

    std::sort(T.begin(), T.end(), std::greater<int>());

    // std::cout << "T: ";
    // for (int i = 0; i < T.size(); i++) {
    //     std::cout << T[i] << " ";
    // }
    int output = 0;
    if (h % T.back() != 0 || w % T.back() != 0) {
        std::cout << -1 << std::endl;
        return 0;
    }

    std::cout << find_top_score(h, w);
    
    return 0;
}