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#ifdef LOCAL
class SS {
   int x[0];
};
extern "C" SS __sanitizer_print_stack_trace();
#define __last_state                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   \
   }                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   \
   ;                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   \
   SS x {                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              \
      __sanitizer_print_stack_trace()
#endif
#include <bits/stdc++.h>
#include <chrono>
#include <ext/pb_ds/assoc_container.hpp>
#include <math.h>
using namespace __gnu_pbds;
using namespace std;
#define endl "\n"
#define mp make_pair
#define st first
#define nd second
#define pii pair<int, int>
#define vi vector<int>
#define pb push_back
#define _upgrade ios_base::sync_with_stdio(0), cout.setf(ios::fixed), cout.precision(10), cin.tie(0), cout.tie(0);
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define fwd(i, a, b) for (int i = (a); i < (b); ++i)
#define trav(a, x) for (auto &a : x)
#define all(c) (c).begin(), (c).end()
#define sz(X) (int)((X).size())
typedef long double ld;
typedef unsigned long long ull;
typedef long long ll;
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> indexed_set; // find_by_order(x) <-returns x-th element   order_of_key(x) <- returns order of element x
// mt19937_64 gen(chrono::steady_clock::now().time_since_epoch().count());uniform_int_distribution<int> distr(0, 1e9);auto my_rand = bind(distr, gen); // my_rand() zwraca teraz liczbe z przedzialu [a, b]
#ifdef LOCAL
ostream &operator<<(ostream &out, string str) {
   for (char c : str)
      out << c;
   return out;
}
template <class L, class R> ostream &operator<<(ostream &out, pair<L, R> p) { return out << "(" << p.st << ", " << p.nd << ")"; }
template <class L, class R, class S> ostream &operator<<(ostream &out, tuple<L, R, S> p) {
   auto &[a, b, c] = p;
   return out << "(" << a << ", " << b << ", " << c << ")";
}
template <class T> auto operator<<(ostream &out, T a) -> decltype(a.begin(), out) {
   out << '{';
   for (auto it = a.begin(); it != a.end(); it = next(it))
      out << (it != a.begin() ? ", " : "") << *it;
   return out << '}';
}
void dump() { cerr << "\n"; }
template <class T, class... Ts> void dump(T a, Ts... x) {
   cerr << a << ", ";
   dump(x...);
}
#define debug(...) cerr << "[" #__VA_ARGS__ "]: ", dump(__VA_ARGS__)
#else
#define debug(...) ;
#endif

#define int long long

pair<int, vector<pii>> split(pii p, int d) {
   auto [a, b] = p;

   auto ap = a % d;
   auto bp = b % d;

   vector<pii> res;
   vector<pii> _tmp = {{ap, bp}, {a - ap, bp}, {ap, b - bp}};

   for (auto [x, y] : _tmp)
      if (x and y)
         res.push_back({x, y});
   return {(a / d) * (b / d), res};
}

int32_t main() {
   _upgrade;
   int h, w, n;
   cin >> h >> w >> n;

   vi D(n);
   for (auto &d : D)
      cin >> d;
   reverse(all(D));
   int res = 0;

   map<pii, int> M = {{{h, w}, 1}};

   for (auto d : D) {
      map<pii, int> Mp;
      for (auto [p, c1] : M) {
         auto [c2, v] = split(p, d);
         for (auto x : v)
            Mp[x] += c1;
         res += c1 * c2;
      }
      swap(M, Mp);
   }
   cout << (sz(M) ? -1 : res) << endl;
}