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#include <iostream>
#include <string>
#include <bitset>
#include <utility>
#include <vector>
#include <array>
#include <map>
#include <algorithm>
#include <deque>
#include <unordered_set>
#include <unordered_map>
#include <memory>

using ll = long long int;
ll solve_line(int h, int w, std::vector<int> const& images, int end);
ll solve(int h, int w, std::vector<int> const& images, int end);



ll solve_line(int h, int w, std::vector<int> const& images, int end)
{
    int value = images.at(end);
    int count = w / value;
    int rest = w % value;

    return count + solve(std::min(rest, h), std::max(rest, h), images, end-1);
}

ll solve(int h, int w, std::vector<int> const& images, int end)
{
    if (h <= 0 || w <= 0 || end < 0)
        return 0;
    
    while(images.at(end)>h)
        end--;

    int value = images.at(end);
    int line_count = h / value;
    ll tmp = line_count * solve_line(value, w, images, end);

    if (h % value == 0)
        return tmp; 

    return tmp + solve(h % value, w, images, end);
}

int main()
{
    int h,w;
    std::cin >> h >> w;
    int n;
    std::cin>>n;
    std::vector<int> images;
    for(int i = 0; i<n; ++i)
    {
        int tmp;
        std::cin >> tmp;
        images.push_back(tmp);
    }
    int smallest = images.front();
    if (h % smallest != 0 || w % smallest != 0)
    {
        std::cout << "-1" << std::endl;
    }
    else
    {
        int small = std::min(h,w);
        int big = std::max(h,w);
        std::cout << solve(small, big, images, images.size() - 1) << std::endl;
    }

    return 0;
}