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#include <iostream>
#include <vector>
#include <string>
#include <numeric>
#include <algorithm>
#include <map>
#include <set>
#include <queue>
#include <stack>

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

    long long h, w;
    long long r = 0;

    std::stack<std::pair<long long, long long>> wall;
    std::cin >> h >> w;

    wall.push(std::make_pair(h, w));

    long long n;

    std::cin >> n;

    std::vector<long long> dimensions(n);

    for (int i = 0; i < n; i++)
    {
        std::cin >> dimensions[i];
    }

    while (!wall.empty())
    {
        std::pair<long long, long long> current = wall.top();
        wall.pop();

        bool found = false;
        long long height = current.first;
        long long width = current.second;

        for (int i = n - 1; i >= 0; i--)
        {
            if (dimensions[i] <= height && dimensions[i] <= width)
            {
                long long inHeight = height / dimensions[i];
                long long inWidth = width / dimensions[i];

                r += inHeight * inWidth;

                if (inHeight * dimensions[i] < height)
                {
                    wall.push(std::make_pair(height - inHeight * dimensions[i], width));

                    if (inWidth * dimensions[i] < width)
                    {
                        wall.push(std::make_pair(height - (height - inHeight * dimensions[i]), width - inWidth * dimensions[i]));
                    }
                }
                else if (inWidth * dimensions[i] < width)
                {
                    wall.push(std::make_pair(height, width - inWidth * dimensions[i]));
                }

                found = true;
                break;
            }
        }

        if (!found)
        {
            std::cout << -1 << std::endl;
            return 0;
        }
    }

    std::cout << r << std::endl;
}