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#include <bits/stdc++.h> 

using namespace std;

typedef long long ll;


ll res(ll h, ll w, ll n, ll d[]) {
    if (h == 0 || w == 0) {
        return 0;
    }

    ll p = 1;
    ll k = n;
    ll m;
    while (p < k) {
        m = (p + k + 1) / 2;
        if (d[m] <= min(h, w)) {
            p = m;
        }
        else {
            k = m - 1;
        }
    }

    ll ind = p;
    
    return (h / d[ind]) * (w / d[ind]) + res(h % d[ind], d[ind] * (w / d[ind]), ind - 1, d) + res(d[ind] * (h / d[ind]), w % d[ind], ind - 1, d) + res(h % d[ind], w % d[ind], ind - 1, d);
}

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

    int h, w;
    cin >> h >> w;
    
    int n;
    cin >> n;
    
    ll d[n + 1];
    for (int i = 1; i <= n; i++) {
        cin >> d[i]; 
    }

    if (h % d[1] == 0 && w % d[1] == 0) {
        h /= d[1];
        w /= d[1];
        for (int i = n; i > 0; i--) {
            d[i] /= d[1];
        }
        cout << res(h, w, n, d) << "\n";
    }
    else {
        cout << "-1\n";
    }

    return 0;
}