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#include<bits/stdc++.h>
using namespace std;

void solve() {
    long long h, w; cin >> h >> w;
    long long n; cin >> n;
    vector<long long> t(n);
    for(int i = 0; i < n; i++) {
        cin >> t[i];
    }
    if(h % t[0] != 0 || w % t[0] != 0) {
        cout << -1 << "\n";
        return;
    }
    reverse(t.begin(), t.end());
    long long ans = 0LL;
    long long area = 0LL;
    for(int i = 0; i < n; i++) {
        long long new_area = (h - (h % t[i])) * (w - (w % t[i])) - area;
        ans += new_area / (t[i] * t[i]);
        area += new_area;
    }
    cout << ans << "\n";
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    solve();
    return 0;
}