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#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
using namespace __gnu_pbds;
using namespace std;
#define pb push_back
#define st first
#define nd second
typedef long long ll;
typedef long double ld;
const ll I = 1000LL * 1000LL * 1000LL * 1000LL * 1000LL * 1000LL;
const ll M = 1000LL * 1000LL * 1000LL + 7LL;
const int N = 1000 * 1000 + 7;
ll tab[32];

void Solve()
{
    int n;
    ll h, w, lh, lw, ans = 0LL;
    cin >> h >> w;
    cin >> n;
    for(int i = 1; i <= n; ++i)
        cin >> tab[i];
    if(!(h % tab[1] == 0 && w % tab[1] == 0))
    {
        cout << -1 << "\n";
        return;
    }
    lh = h; lw = w;
    for(int i = n; i >= 1; --i)
    {
        ll a = lh / tab[i], b = lw / tab[i];
        ans += ((h - lh) / tab[i]) * b;
        ans += ((w - lw) / tab[i]) * a;
        ans += a * b;
        if(b > 0 || ((w - lw) / tab[i]) > 0)
            lh -= a * tab[i];
        if(a > 0 || ((h - lh) / tab[i]) > 0)
            lw -= b * tab[i];
    }
    cout << ans << "\n";
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    //int t; cin >> t;
    //while(t--)
        Solve();

    return 0;
}