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#include<bits/stdc++.h>
 
#define pb push_back
#define pob pop_back
#define eb emplace_back
#define fi first
#define se second
#define turbo ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define mid ((l+r)/2)
#define midLL ((l+r)/2LL)
#define mide ((lo+hi)/2)
#define mideLL ((lo+hi)/2LL)
#define endl '\n'
#define shandom_ruffle random_shuffle
#define lowbit(x) x&(-x)
#define bits(x) __builtin_popcount(x) 
#define mins(se) (*se.begin())
#define maxs(se) (*--se.end())
#define all(x) x.begin(), x.end()
#define log2_floor(i) (i ? __builtin_clzll(1) - __builtin_clzll(i) : -1)
#define siz(cont) ((int)cont.size())
#define each(it, cont) for(auto &it : cont)

using ll = long long;
using pii = std::pair<int, int>;
using pll = std::pair<ll, ll>;
using pli = std::pair<ll, int>;
using pil = std::pair<int, ll>;
using vll = std::vector<pll>;
using vii = std::vector<pii>;
using vi = std::vector<int>;
using vl = std::vector<ll>;
using vvl = std::vector<vl>;
using vvi = std::vector<vi>;
using vvii = std::vector<vii>;
using vli = std::vector<pli>;
using vvli = std::vector<vli>;
using vil = std::vector<pil>;
using vvil = std::vector<vil>;
using vc = std::vector<char>;
using vvc = std::vector<std::vector<char>>;
using vb = std::vector<bool>;
using vvb = std::vector<vb>;

void err() { std::cout << "\n"; fflush(stdout); }
template<class T, class... Ts>
void err(T arg, Ts &... args) {
    std::cout << arg << ' ';
    err(args...);
}

using namespace std;

set<ll> sd;

map<pll, ll> ch;

ll miel(ll h, ll w) {
    if (!h or !w)
        return 0;
    if (w < h)
        swap(h, w);
    if(ch[{h,w}])
        return ch[{h,w}];
    ll best = h*w;
    for (auto d : sd) {
        if (d > h or d > w)
            break;
        ll res = (h/d)*(w/d);
        res += miel(h%d, w%d);
        res += miel(h-h%d, w%d);
        res += miel(h%d, w-w%d);
        best = min(best, res);
    }
    return ch[{h, w}] = best;
}


signed main() {
    turbo;
    ll h, w, n;
    cin >> h >> w >> n;
    while (n--) {
        int x; cin >> x;
        sd.insert(x);
    }
    if (h%mins(sd) or w%mins(sd)) {
        cout << -1;
        return 0;
    }
    cout << miel(h, w);
}