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#include <bits/stdc++.h>
#define all(x) (x).begin(),(x).end()
using namespace std;

using ll = long long;
using ld = long double;

#define int ll
#define sz(x) ((int)(x).size())

using pii = pair<int,int>;
using tii = tuple<int,int,int>;

signed main() {
  cin.tie(0) -> sync_with_stdio(0);
  int n, H, W;
  cin >> H >> W >> n;
  
  vector<int> v(n);
  for(auto &x : v) cin >> x;
  reverse(all(v));
  
  ll rez = 0;
  
  while(sz(v) > 1) {
    int a = v.back();
    v.pop_back();
    //cerr << a << '\n';
    if(H % a != 0 || W % a != 0) {
      cout << "-1\n";
      return 0;
    }
    //cerr << v.back() << '\n';
    int h = H % v.back(), w = W % v.back();
    rez += (h * W + w * H - h * w) / (a * a);
    
    //cerr << h << ' ' << w << ' ' << H << ' ' << W << '\n';
    
    H -= h;
    W -= w;
  }
  
  if(H % v.back() != 0 || W % v.back() != 0) {
    cout << "-1\n";
    return 0;
  }
  
  rez += H * W / (v.back() * v.back());
  
  cout << rez << '\n';
}

/**
  Anul asta nu se da centroid
  -- Rugaciunile mele
*/