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

#define LL long long

using namespace std;

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

    LL w, h;
    cin >> w >> h;

    int n;
    cin >> n;
    vector<LL> d(n);
    for (int i = 0; i < n; ++i) cin >> d[i];
    for (int i = n - 1; i; --i) d[i] /= d[i - 1];

    LL res = 0ll;
    for (int i = 0; i < n; ++i)
    {
        res += (w % d[i]) * h + (h % d[i]) * w - (w % d[i]) * (h % d[i]);
        w /= d[i];
        h /= d[i];

        if (!i && res)
        {
            cout << "-1";
            return 0;
        }
    }

    res += w * h;
    cout << res;
}