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#pragma GCC optimize("O3")
#include <bits/stdc++.h>
using namespace std;

#define FOR(i, n) for (int i = 0; i < n; i++)
#define f first
#define s second
#define pb push_back
#define all(s) s.begin(), s.end()
#define sz(s) (int)s.size()

using vi = vector<int>;
using ll = long long;

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

    int n;
    cin >> n;
    vi obrazy(n);
    FOR(i, n) cin >> obrazy[i];
    reverse(all(obrazy));

    if (w % obrazy.back() or h % obrazy.back())
    {
        cout << -1 << '\n';
        return;
    }

    function<ll(int a, int b)> policz = [&](int a, int b) -> ll {
        if (a == 0 or b == 0)
            return 0;
        
        for (auto & u : obrazy)
        {
            if (u <= a and u <= b)
            {
                return 1ll * (a / u) * (b / u) + policz(a % u, b - b % u) + policz(a - a % u, b % u) + policz(a % u, b % u);
            }
        }
        assert(false);
    }; 
    cout << policz(w, h) << '\n';
}

int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);

    int tests = 1;
    // cin >> tests;

    for (int test = 1; test <= tests; test++)
    {
        solve();
    }

    return 0;
}