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#include <iostream>
#include <algorithm>
#include <map>
#include <vector>
#include <stack>

using namespace std;


long long sol(vector<long long>& d, long long h, long long w) {
  if (w == 0 || h == 0) {
    return 0;
  }

  for (auto it = d.rbegin(); it != d.rend(); ++it) {
    if (*it <= h && *it <= w) {
      long long a = (w / *it) * sol(d, h % *it, *it);
      long long b = (h / *it) * sol(d, *it, w % *it);
      long long c = sol(d, h % *it, w % *it);
      long long result = (a > -1 && b > -1 && c > -1) ? ((h / *it) * (w / *it)) + a + b + c : -1;
      return result;
    }
  }

  return -1;
}

int main() {
  std::ios_base::sync_with_stdio(false);
  std::cin.tie(NULL);

  int h, w;
  cin >> h >> w;

  int n;
  cin >> n;

  vector<long long> d(n);
  for (int i = 0; i < n; ++i) {
    cin >> d[i];
  }

  cout << sol(d, h, w) << "\n";
  return 0;
}