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#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

#define rep(i, j) for (int i = 0; i < (int)(j); i++)
#define st first
#define nd second

void TEST([[maybe_unused]] int testcase_i) {
  int h, w, n;
  cin >> h >> w >> n;
  vector<int> v(n);
  rep(i, n) {
    cin >> v[i];
  }

  if (h % v[0] or w % v[0]) {
    cout << -1;
    return;
  }

  reverse(v.begin(), v.end());

  ll ans = 0;

  vector<pair<int, int>> pieces{{h, w}};
  while (pieces.size()) {
    auto [ph, pw] = pieces.back();
    pieces.pop_back();
    if (ph == 0 or pw == 0) {
      continue;
    }
    rep(i, n) {
      if (min(ph, pw) < v[i]) {
        continue;
      }
      ans += (ll)(ph / v[i]) * (ll)(pw / v[i]);
      pieces.push_back({ph, pw % v[i]});
      pieces.push_back({ph % v[i], (pw / v[i]) * v[i]});
      break;
    }
  }

  cout << ans;
}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);

  cout << setprecision(20) << fixed;

  // srand(time(NULL));

  int t = 1;
  // cin >> t;
  rep(i, t) {
    TEST(i);
  }

  return 0;
}