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

#define mp(a,b) make_pair(a,b)
#define ff first
#define setp(a) setprecision(a)<<fixed
#define ss second
#define fori(v) for(int i=0; i<v; i++)
#define forj(v) for(int j=0; j<v; j++)
#define fork(v) for(int k=0; k<v; k++)
#define forl(v) for(int l=0; l<v; l++)
#define fort(v) for(int t=0; t<v; t++)
#define forz(v) for(int z=0; z<v; z++)
#define forx(v) for(int x=0; x<v; x++)
#define fory(v) for(int y=0; y<v; y++)
#define ll long long
#define lll __int128
#define ld long double
#define pb(a) push_back(a)
// #define cout out
// #define cin in
ll inf = (ll)pow(10,18);
ll modulo = 998244353;
ld eps = 1e-13;
ifstream in;
ofstream out;

ll solve(ll w, ll h, vector<ll>& all){
      if(!w || !h){
            return 0;
      }
      for(auto& el : all){
            if(el <= w && el <= h){
                  ll how = (w/el) * (h/el);
                  ll m1 = w % el, m2 = h % el;
                  ll ans = how + solve(m1, h-m2, all) + solve(w-m1, m2, all) + solve(m1, m2, all);
                  if(ans < 0){
                        ans = -inf;
                  }
                  return ans;
            }
      }
      return -inf;
}

void deal(){
      ll w, h;
      cin>>w>>h;
      ll n;
      cin>>n;
      vector<ll> all(n);
      fori(n){
            cin>>all[i];
      }
      
      sort(all.rbegin(), all.rend());
      
      ll ans = solve(w, h, all);
      
      if(ans < 0){
            ans = -1;
      }
      
      cout<<ans;
}

int main(){
	cin.tie(0);
	ios_base::sync_with_stdio(0);
	deal();
}