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#if defined(EMBE_DEBUG) && !defined(NDEBUG)
#include "embe-debug.hpp"
#else
#define LOG_INDENT(...) do {} while (false)
#define LOG(...) do {} while (false)
#define DUMP(...) do {} while (false)
#endif

#include <algorithm>
#include <iostream>
#include <utility>
#include <vector>
using namespace std;

namespace {

long long solve(int h, int w, vector<int> sizes)
{
  int n = sizes.size();
  sizes.push_back(1'000'111'222);

  long long res = 0;

  for (int i = 0; i < n; ++i) {
    int cur = sizes[i];
    int next = sizes[i + 1];
    int dh = h % next;
    int dw = w % next;
    if (dh % cur != 0 || dw % cur != 0) return -1;
    h -= dh;
    res += static_cast<long long>(dh / cur) * (w / cur);
    w -= dw;
    res += static_cast<long long>(dw / cur) * (h / cur);
  }

  return res;
}

}

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

  int h, w;
  cin >> h >> w;
  int n;
  cin >> n;
  vector<int> sizes(n);
  for (auto& s: sizes) {
    cin >> s;
  }

  cout << solve(h, w, move(sizes)) << endl;

  return 0;
}