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#include <bits/stdc++.h>

using namespace std;

unsigned long long int wynik = 0;

void f (unsigned long long int k, unsigned long long int h, unsigned long long int tab[], int n, int itr) {
    //cout << k << ' ' << h << ' ' << n << ' ' << itr << "\n";
    while (tab[itr] > k || tab[itr] > h) itr--;
    wynik = wynik + (h / tab[itr]) * (k / tab[itr]);

    if (h % tab[itr] != 0) {
        f (tab[itr] * (k / tab[itr]), h % tab[itr], tab, n, itr);
    }

    if (k % tab[itr] != 0) {
        f (k % tab[itr], tab[itr] * (h / tab[itr]), tab, n, itr);
    }

    if (h % tab[itr] != 0 && k % tab[itr] != 0) {
        f (k % tab[itr], h % tab[itr], tab, n, itr);
    }
    return;
}

int main()
{
    cin.tie(0);
    ios_base::sync_with_stdio(0);
    unsigned long long int h, w;
    cin >> h >> w;
    int n;
    cin >> n;
    unsigned long long int tab[n];
    for (int i = 0; i < n; i++) {
        cin >> tab[i];
    }

    if (h % tab[0] != 0) {
        cout << -1;
        return 0;
    }
    if (w % tab[0] != 0) {
        cout << -1;
        return 0;
    }

    int itr = n-1;
    f(w, h, tab, n, itr);
    cout << wynik;
    return 0;
}