#include <bits/stdc++.h>

using namespace std;

vector<long long >g;

long long suma=0;

void f2(long long h,long long w,long long i)
{
    if(h==0)return ;
    long long res1=h%g[i];
    suma+=(w/g[i])*(h/g[i]);
   // cout<<suma<<endl;
    f2(res1,w,i-1);
}


void f1(long long h,long long w,long long i)
{
    if(w==0)return ;
    long long res1=w%g[i],res2=h%g[i];
   // cout<<res1<<" "<<res2<<"XD"<<endl;
    suma+=(w/g[i])*(h/g[i]);
   // cout<<suma<<endl;
    f2(res2,w-res1,i-1);
    f1(h,res1,i-1);
}


int main()
{
    long long h,w,n,a;
    cin>>h>>w>>n;
    for(int i=0;i<n;i++)
    {
        cin>>a;
        g.push_back(a);
    }
    if(h%g[0]!=0 || w%g[0]!=0)
    {
        cout<<-1;
        return 0;
    }
    f1(h,w,n-1);
    cout<<suma;
}