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#include<bits/stdc++.h>
 
//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("trapv")
 
#define st first
#define nd second
#define pb push_back
#define pp pop_back
#define eb emplace_back
#define mp(a, b) make_pair(a, b)
#define all(x) (x).begin(), (x).end()
#define rev(x) reverse(all(x))
#define sor(x) sort(all(x))
#define sz(x) (int)(x).size()
#define rsz(x) resize(x)
 
using namespace std;
 
///~~~~~~~~~~~~~~~~~~~~~~~~~~
 
template <typename H, typename T> 
ostream& operator<<(ostream& os, pair<H, T> m){
	return os <<"("<< m.st<<", "<<m.nd<<")";
}
template <typename H> 
ostream& operator<<(ostream& os, vector<H> V){
	os<<"{";
	for(int i=0; i<V.size(); i++){
		if(i)os<<" ";
		os<<V[i];
	}
	os<<"}";
	return os;
}
 
void debug(){cerr<<"\n";}
template <typename H, typename... T>
void debug(H h, T... t) {cerr<<h; if (sizeof...(t)) cerr << ", "; debug(t...);}
#define deb(x...) cerr<<#x<<" = ";debug(x);
 
///~~~~~~~~~~~~~~~~~~~~~~~~~
 
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<pii > vii;
typedef vector<ll> vl;
typedef vector<pll> vll;
typedef string str;
 
#define BOOST ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
 
mt19937 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
 
const int N=2e5+5, INF=1e9+5, mod=1e9+7;

int main(){
	//BOOST;
	int w, h;
	cin>>w>>h;
	int n;
	cin>>n;
	vi V(n);
	ll ans=0;
	for(int &i:V){
		cin>>i;
	}
	if(h%V[0] || w%V[0])cout<<"-1\n";
	else{
		for(int i=1; i<n; i++){
			deb(h, w);
			int w2=(w/V[i]*V[i]);
			int h2=(h/V[i]*V[i]);
			ans+=(w*1ll*h-w2*1ll*h2)/V[i-1]/V[i-1];
			w=w2;h=h2;
		}
		deb(w, h);
		ans+=(w/V.back())*1ll*(h/V.back());
		cout<<ans<<"\n";
	}
}