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#include <bits/stdc++.h>
using namespace std;
using ll = long long;

#ifdef LOCAL
#include "debug.h"
#else
#define debug(...)
#endif


ll dp(int h, int w, const vector<int>& a) {
  for (int d : a) {
    if (d <= min(h,w)) {
      return (ll)(w/d)*(h/d) + dp(h,w%d,a) + dp(h%d,w-w%d,a);
    }
  }
  return 0;
}

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

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

  int n; cin >> n;
  vector<int> a(n);
  for (int& x : a) cin >> x;
  
  if (h % a[0] || w % a[0]) {
    cout << -1 << '\n';
    return 0;
  }
  
  reverse(a.begin(), a.end());
  cout << dp(h, w, a) << '\n';
}