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

int h, w, n;
std::vector<int> d;

void input() {
    std::cin >> h >> w >> n;
    int x;
    for (int i = 0; i < n; i++) {
        std::cin >> x;
        d.push_back(x);
    }
    std::reverse(d.begin(), d.end());
}

long long solve(int x, int y) {
    int m = std::min(x, y);
    if (m == 0) {
        return 0;
    }

    int i = 0;
    while (i < n && d[i] > m) {
        i++;
    }
//    std::cout << x << " " << y << " " << m << " " << d[i] << "\n";

    if (i == n) {
        return -1;
    }

    long long used_horizontal = x / d[i];
    long long used_vertical = y / d[i];
    long long result = used_horizontal * used_vertical;
    long long x_diff = used_horizontal * d[i];
    long long y_diff = used_vertical * d[i];

    long long r1 = solve(x - x_diff, y_diff);
    long long r2 = solve(x_diff, y - y_diff);
    long long r3 = solve(x - x_diff, y - y_diff);
    if (r1 == -1 || r2 == -1 || r3 == -1) {
        return -1;
    }

    return result + r1 + r2 + r3;
}

int main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(nullptr);

    input();
    std::cout << solve(w, h) << "\n";

    return 0;
}