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#include<bits/stdc++.h>
using namespace std;
#define pb push_back
#define eb emplace_back
//mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
#define mod 998244353
#define int long long
#define inf 0x3f3f3f3f
#define INF 0x3f3f3f3f3f3f3f3f
inline int read()
{
	char ch=getchar(); int nega=1; while(!isdigit(ch)) {if(ch=='-') nega=-1; ch=getchar();}
	int ans=0; while(isdigit(ch)) {ans=ans*10+ch-48;ch=getchar();}
	if(nega==-1) return -ans;
	return ans;
}
void print(vector<int> x){for(int i=0;i<(int)x.size();i++) printf("%d%c",x[i]," \n"[i==(int)x.size()-1]);}
#define N 35
int a[N];
signed main()
{
	int n,m; cin>>n>>m;
	int k; cin>>k;
	for(int i=1;i<=k;i++) a[i]=read();
	sort(a+1,a+k+1,greater<int>());
	if(n%a[k]!=0||m%a[k]!=0) cout<<"-1\n";
	else
	{
		int ans=0,p=0,q=0;
		for(int i=1;i<=k;i++)
		{
			ans+=(n/a[i])*(m/a[i])-(p/a[i])*(q/a[i]);
			p=n/a[i]*a[i],q=m/a[i]*a[i];
		}
		cout<<ans<<endl;
	}
	return 0;
}