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#include <bits/stdc++.h>
#define FOR(i, a, b) for(int i =(a); i < (b); ++i)
#define re(i, n) FOR(i, 1, n)
#define ford(i, a, b) for(int i = (a); i >= (b); --i)
#define rep(i, n) for(int i = 0;i <(n); ++i)
#define all(x) (x).begin(), (x).end()
#define sz(x) (int) (x).size()

using namespace std;

typedef long long ll;
typedef pair<ll, ll> pll;
typedef pair<int, int> pii;
typedef vector<ll> vll;
typedef vector<int> vi;
typedef vector<pll> vpll;
typedef vector<pii> vpii;

const ll inf = 1e18;
const int N = 5005;
const int mod = 1e9 + 7;
ll h, w;
int n;
vector<ll> in; 

ll solve(int i, ll h, ll w)
{
    if (h == 0 || w == 0) return 0;
    if (in[i] > h || in[i] > w) return solve(i-1, h, w);
    ll dupa = (h/in[i]) * (w/in[i]);
    ll res1 = solve(i-1, h % in[i], w - (w % in[i]));
    ll res2= solve(i - 1, h, w % in[i]);
    return dupa + res1 + res2;
}

int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cin >> h >> w >> n;
    in.resize(n);
    rep(i, n)
    {
        cin >> in[i];
    }
    if (h % in[0] != 0 || w % in[0] != 0)
    {
        cout << "-1\n";
        return 0;
    }
    cout << solve(n-1, h, w) << '\n';
}