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#include <iostream>
#include <vector>
#include <algorithm>
#include <math.h>
#include <set>
#include <map>
#include <stack> 
#include <queue> 
#include <iomanip>              
using namespace std;
typedef long long ll;
const int N = 2e2 + 10;
const int P = 101;
const ll MLL = 1e18;
const ll mod = 998244353;
const ll LOG = 31;
vector<ll> v;
ll h , w;
int n;
ll ans = 0;											// robimy lewa i prawa a potem rekursywnie do przedialu na dole 
												
												
bool ne() {
	while(v.size() > 0 && v[v.size() - 1] > min(h , w)) v.pop_back();
	if(v.size() == 0) {
		return false;
	}
	 n = v.size();
	 ll last = v[n - 1];
	 ans += (h / last) * (w / last);
	 ll hfil = h - (h % last), wfil = w - (w % last);
	 ll hmod = h % last , wmod = w % last;
	 for(int i = n - 2; i >= 0; i--) {
		 if(v[i] > hmod) continue;
		 ll use = hmod / v[i];
		 hmod %= v[i];
		 ans += wfil/v[i] * use;
	 }
	  for(int i = n - 2; i >= 0; i--) {
		 if(v[i] > wmod) continue;
		 ll use = wmod / v[i];
		 wmod %= v[i];
		 ans += hfil/v[i] * use;
	 }
	 if(wmod != 0 || hmod != 0) {
		 return false;
	 }
	 h = h % last , w = w % last;
	 return true;
}
												
												
void solve() {

	cin >> h >> w;
	cin >> n;
	v.resize(n);
	for(int i =0 ; i < n; i++) cin >> v[i];
	bool ok = true;
	while(w != 0 && h != 0) {
		bool f = ne();
		if(!f) {
			ok = false;
			break;
		}
	}
	
	if(ok) cout << ans << endl;
	else cout << -1 << endl;
}

int main()
{
	ios_base::sync_with_stdio(false);
    cin.tie(0);
	
    solve();


    return 0;
}