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#include <bits/stdc++.h>

#define int long long
#define ll long long
#define ld long double
#define endl '\n'
#define st first
#define nd second
#define pb push_back
#define sz(x) (int)(x).size()
#define all(x) (x).begin(), (x).end()
using namespace std;
int inf=1000000000000000007;
int mod=1000000007;
int mod1=998244353;

const int N=37;

int d[N];

signed main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int h,w,n;
    cin>>h>>w;
    cin>>n;
    for(int i=1;i<=n;i++) cin>>d[i];
    if(h%d[1]!=0||w%d[1]!=0) 
    {
		cout<<-1;
		return 0;
	}
	int ans=0;
	h/=d[1];
	w/=d[1];
	for(int i=n;i>1;i--) d[i]/=d[i-1];
	for(int i=1;i<n;i++)
	{
		int a=h%d[i+1],b=w%d[i+1];
		ans+=a*w+b*h-a*b;
		h-=a;
		w-=b;
		h/=d[i+1];
		w/=d[i+1];
	}
	ans+=h*w;
	cout<<ans;
    return 0;
}