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

typedef long long ll;
typedef unsigned int ui;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<long long> vll;
const ll LINF = 1e18;
const int INF = 1e9;

ll solve(int h, int w, const vi &d, int prev_used) {
    if (h == 0 || w == 0) {
        return 0;
    }

    int el_to_use = prev_used - 1;
    while (el_to_use > 0 && (d[el_to_use] > h || d[el_to_use] > w)) {
        --el_to_use;
    }

    int d_used = d[el_to_use];
    ll h_fit = h / d_used;
    ll w_fit = w / d_used;

    assert(h_fit && w_fit);

    return h_fit * w_fit
        + solve(h_fit * d_used, w - w_fit * d_used, d, el_to_use)
        + solve(h - h_fit * d_used, w_fit * d_used, d, el_to_use)
        + solve(h - h_fit * d_used, w - w_fit * d_used, d, el_to_use);
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int h, w, n;
    cin >> h >> w >> n;

    vi d(n);
    for (int i = 0; i < n; ++i) {
        cin >> d[i];
    }
    sort(d.begin(), d.end());

    if (h % d[0] != 0 || w % d[0] != 0) {
        cout << "-1" << endl;
    } else {
        cout << solve(h, w, d, n) << endl;
    }

    return 0;
}