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#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

ll countFit(ll w, ll h, ll s, ll &cornerX, ll &cornerY) {
    ll wX = w / s, wY = h / s;

    cornerX = wX * s;
    cornerY = wY * s;

    return wX * wY;
}

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

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

    sort(v.rbegin(), v.rend());

    ll cornerX = 0, cornerY = 0;
    ll result = countFit(w, h, v[0], cornerX, cornerY);

    ll id = 1;
    while ((cornerX < w || cornerY < h) && id < v.size()) {
        ll tempX, tempY;
        result += countFit(w, h, v[id], tempX, tempY) - countFit(cornerX, cornerY, v[id], cornerX, cornerY);
        cornerX = tempX;
        cornerY = tempY;
        id++;
    }

    if (cornerX < w || cornerY < h) cout << -1;
    else cout << result;
}