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#include<bits/stdc++.h>
#define poly vector<int>
#define IOS ios::sync_with_stdio(false)
#define ll long long
#define mp make_pair
#define mt make_tuple
#define pa pair < int,int >
#define fi first
#define se second
#define inf 1e18
#define mod 998244353
#define sz(x) (int)((x).size())
#define int ll
//#define N
using namespace std;
int h,w,n,a[35];
void BellaKira()
{
	cin>>h>>w;
	cin>>n;
	for (int i=1;i<=n;i++)
		cin>>a[i];
	int lst=0,ans=0;
	for (int i=n;i>=1;i--)
	{
		int x=h/a[i],y=w/a[i];
		ans+=x*y-lst*(a[i+1]/a[i])*(a[i+1]/a[i]);
		// cout<<i<<" "<<x*y-lst*(a[i+1]/a[i])*(a[i+1]/a[i])<<endl;
		lst=x*y;
	}
	if (h%a[1]||w%a[1]) cout<<"-1\n";
	else
		cout<<ans<<'\n';
		
}
signed main()
{
	IOS;
	cin.tie(0);
	int T=1;
	while (T--)
	{
		BellaKira();
	}
}