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#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
const int MAXN=30;

int n;
ll arr[MAXN+5];

ll ans=0;
bool f(ll a, ll b){
	if(a==0 || b==0) return 1;
	if(a > b) swap(a, b);
	int i=n;
	ll na=a, nb=-1;
	ll cb=-1;
	for(i;i>=1;i--){
		if(arr[i] > a) continue;
		nb = (b / arr[i]) * arr[i];
		cb = b % arr[i];
		break;
	}
	for(i;i>=1;i--){
		if(arr[i] > na) continue;
		ans += (na / arr[i]) * (nb / arr[i]);
		na %= arr[i];
	}
	if(na != 0){
		return 0;
	}
	if(!f(a, cb)) return 0;
	return 1;
}

void solve()
{
	ll h,w;
	cin >> h >> w;
	cin >> n;
	for(int i=1;i<=n;i++){
		cin >> arr[i];
	}
	if(!f(h, w)) cout << -1;
	else cout << ans;
}

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