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#include <bits/stdc++.h>

using namespace std;

using ll = long long;
using llu = unsigned long long;

#define int ll

int32_t main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    llu h, w;
    cin >> h >> w;
    llu n;
    cin >> n;
    vector<llu> d(n);
    for (llu i = 0; i < n; i++) {
        cin >> d[i];
    }
    if (h % d[0] || w % d[0]) {
        cout << "-1\n";
        return 0;
    }
    llu ans = h / d[0] * (w / d[0]);
    h /= d[0];
    w /= d[0];
    for (llu i = 1; i < n; i++) {
        llu x = d[i] / d[i-1];
        h /= x;
        w /= x; 
        ans -= h * w * x * x;
        ans += h * w;
    }
    cout << ans << '\n';

}