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#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

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

    ll h, w;
    cin >> h >> w;
    int n;
    cin >> n;
    vector<ll> sizes(n);
    for(int i = 0; i < n; i++) cin >> sizes[i];

    if(h % sizes[0] == 0 && w % sizes[0] == 0) {
        ll cnt = 0;
        ll usedh = 0, usedw = 0;
        for(int i = n - 1; i >= 0; i--) {
            cnt += (h/sizes[i]) * (w/sizes[i]) - (usedh/sizes[i]) * (usedw/sizes[i]);
            usedh += ((h - usedh) / sizes[i]) * sizes[i];
            usedw += ((w - usedw) / sizes[i]) * sizes[i];
        }

        cout << cnt << '\n';
    } else cout << "-1\n";
}