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#include <bits/stdc++.h>

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;

using namespace __gnu_pbds;
template <typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

#define int long long
const long long INF = 1e18 + 10;

int solve(int w, int h, vector<int> v) {
    if (w == 0 || h == 0)
        return 0;

    if ((w % v[0] + h % v[0]) > 0) {
        return INF;
    }

    while (min(w, h) < v.back())
        v.pop_back();

    int chosen = v.back();

    int res = (w / chosen) * (h / chosen);

    return res +
        solve(w % chosen, h - h % chosen, v) +
        solve(w - w % chosen, h % chosen, v) +
        solve(w % chosen, h % chosen, v);
}

void solve(int _id) {
    int w, h;
    cin >> w >> h;

    int n;
    cin >> n;

    vector<int> v(n);
    for (int &i : v) {
        cin >> i;
    }

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

    cout << solve(w, h, v) << "\n";

}

int32_t main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    int t = 1;
//    freopen("../d/1.in","r",stdin);
//    freopen("../wzo.out","w",stdout);

//    cin >> t;

    for (int i = 1; i <= t; i++) {
        solve(i);
    }

    return 0;
}
/*
1
cc
1
1 1

a
ca
cca
 */