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#include <bits/stdc++.h>
#pragma GCC optimize("O3", "unroll-loops")
#define pb emplace_back
#define ins insert
#define mp make_pair
#define ssize(x) (int)x.size()
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define pii pair <int, int>
#define pll pair <ll, ll>
#define pld pair <ld, ld>
#define st first
#define nd second

using namespace std;
using ll = int_fast64_t;
// using lll = __int128_t;
using ld = long double;
const int oo = 1e9 + 7;
const ll ool = 1e18;
const int mod = 1e9 + 7;

void solve(){
    int h, w; cin >> h >> w;
    int n; cin >> n;
    vector <int> tab(n);
    for(auto &i: tab) cin >> i;
    if((w % tab[0]) || (h % tab[0])){
        cout << "-1\n";
        exit(0);
    }
    auto find = [&](int x){
        int l = 0, r = n;
        while(l != r){
            int mid = (l + r) >> 1;
            if(mid == n){
                r = mid;
                continue;
            }
            if(tab[mid] <= x) l = mid + 1;
            else r = mid;
        }
        assert(l);
        return tab[l - 1];
    };
    ll ans = 0;
    function <void(int, int)> rec = [&](int x, int y){
        if(x > y) swap(x, y);
        int k = find(x);
        if(x == k && y == k){
            ++ ans;
            return;
        }
        ll cnt = (x / k);
        k = k * cnt;
        ans += (ll)(cnt * cnt) * (ll)(y / k);
        if(x % k) rec(k * (y / k), x % k);
        if(y % k) rec(k * (x / k), y % k);
        if(x % k && y % k) rec(x % k, y % k);
    };
    rec(w, h);
    cout << ans << '\n';
}

signed main(){
    ios_base::sync_with_stdio(0); cin.tie(0);
    int t; t = 1;
    // cin >> t;
    while(t --) solve();
    return 0;
}