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

int main() {
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    ll h, w, n;
    cin >> h >> w >> n;

    vector <ll> v(n);
    for (int i = 0; i < n; i++)
        cin >> v[i];
    

    if (h % v[0] || w % v[0]) {
        cout << "-1\n";
        return 0;
    }

    ll res = (h / v[0]) * (w / v[0]);
    for (int i = 1; i < n; i++) {
        ll delta = v[i] / v[i - 1];
        ll num = (h / v[i]) * (w / v[i]);

        res -= num * (delta * delta - 1);
    }

    cout << res << "\n";
    
    return 0;
}