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#include "bits/stdc++.h"
using namespace std;
#define all(x) x.begin(),x.end()
template<typename A, typename B> ostream& operator<<(ostream &os, const pair<A, B> &p) { return os << p.first << " " << p.second; }
template<typename T_container, typename T = typename enable_if<!is_same<T_container, string>::value, typename T_container::value_type>::type> ostream& operator<<(ostream &os, const T_container &v) { string sep; for (const T &x : v) os << sep << x, sep = " "; return os; }
#ifdef LOCAL
#include "debug.h"
#else
#define debug(...) 42
#define ASSERT(...) 42
#endif
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int,int> pi;
const int oo = 1e9;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    ll h,w; cin >> h >> w;
    int n; cin >> n;
    vi d(n);
    for(auto& i : d) cin >> i;
    sort(all(d));
    // greedy packing suffices!
    ll ans=0;
    auto rec = [&](auto&& rec, ll h, ll w) -> void {
        if(h>w) swap(h,w);
        if(h==0) return;
        auto it = upper_bound(all(d),h);
        if(it==d.begin()) {
            cout << "-1\n";
            exit(0);
        }
        ll B = *prev(it);
        ll num = (h/B)*(w/B);
        ans+=num;
        rec(rec,h%B,w);
        rec(rec,h-(h%B),w%B);
    };
    rec(rec,h,w);
    cout << ans << '\n';

}