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#include <bits/stdc++.h>

typedef long long int lli;

using namespace std;

#define MAX4 10010
#define MAX5 100010
#define MAX6 1000010

#define FOR(n) for (int i = 0; i < n; i++)

using vti = vector<int>;
using vts = vector<string>;

template <typename T> void dbg(T last) { cout << last << "\n"; }

template <typename T, typename... args> void dbg(T current, args... next) {
  cout << current << ' ';
  dbg(next...);
}

template <typename T> void getInput(vector<T> &v, int n, int x = 0) {
  for (int i = x; i < n; i++) {
    cin >> v[i];
  }
}

//-----------------------------------------------------------------------------------------------//

lli getAns(set<lli> &s, lli w, lli h) {

  // dbg(w, h);
  if (h == 0) {
    return 0;
  }
  lli ans = 0;
  auto it = s.upper_bound(h);
  it--;
  lli val = *it;
  lli k = h / val;
  h -= k * val;
  ans += getAns(s, w, h);
  w -= val;
  ans += k;
  while (w) {
    lli x = min(w, val);
    auto ittmp = s.upper_bound(x);
    ittmp--;
    lli valtmp = *ittmp;
    lli ktmp = w / valtmp;
    ans += k * ktmp * (val / valtmp);
    // dbg("*", w, h, valtmp, ktmp, k * ktmp * (val / valtmp));
    w -= valtmp * ktmp;
  }

  return ans;
}

void solve() {
  int h, w;
  cin >> h >> w;

  if (h > w) {
    swap(h, w);
  }
  int n;
  cin >> n;
  set<lli> s;
  for (int i = 0; i < n; i++) {
    lli a;
    cin >> a;
    s.insert(a);
  }
  // dbg(*s.begin());
  if (h % (*s.begin()) != 0 || w % (*s.begin()) != 0) {
    cout << "-1\n";
    return;
  }
  cout << getAns(s, w, h) << "\n";
}

int main() {
  std::ios::sync_with_stdio(false);

  solve();

  return 0;
}