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#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#define st first
#define nd second
typedef long long ll;
using namespace std;




int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    ll h, w; cin >> h >> w;
    if (h > w) swap(h, w);
    int n; cin >> n;
    vector<ll> o(n);
    for (int i = 0; i < n; i++) cin >> o[i];
    sort(o.begin(), o.end());
    if (h%o[0] || w%o[0]) {
        cout << "-1\n";
        return 0;
    }
    ll ans = 0;
    for (int i = 1; i < n; i++) {
        ll a = h%o[i], b = w%o[i];
        ans += ((a/o[i-1])*(w/o[i-1])) + ((b/o[i-1])*(h/o[i-1])) - ((a/o[i-1])*(b/o[i-1]));
        h -= a;
        w -= b;
    }
    ans += (h/o[n-1]) * (w/o[n-1]);
    cout << ans << '\n';
}