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

using namespace std;

#define ull unsigned long long
#define ULL_MAX std::numeric_limits<ull>::max()

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    ull h, w;
    cin >> h >> w;
    int n;
    cin >> n;
    ull d;
    ull res;
    cin >> d;
    if (w % d != 0 || h % d != 0) {
        cout << -1;
        return 0;
    }
    res = (w / d) * (h / d);
    w /= d;
    h /= d;
    for (int i = 1; i < n; i++) {
        int prevD = d;
        cin >> d;
        int artD = d / prevD;

        res -= (w / artD) * (h / artD) * (artD * artD - 1);

        w /= artD;
        h /= artD;
    }
    cout << res;
    return 0;
}