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#include <bits/stdc++.h>
#define pb push_back
#define ull unsigned long long
using namespace std;
ull n,w,h,z;
ull wyn, s,a,b;
vector <ull> v;
int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
	cin>>h>>w>>n;
	for (int i=0; i<n; i++){
        cin>>z;
        v.pb(z);
	}
    if (h%v[0]!=0 || w%v[0]!=0){
        cout<<"-1";
        return 0;
    }
    wyn=(h/v[0])*(w/v[0]);
	for (int i=1; i<v.size(); i++){
        s=(v[i]*v[i])/(v[i-1]*v[i-1]);
        a=w/v[i];
        b=h/v[i];
        wyn-=((s-1)*(a*b));
	}
    cout<<wyn;
    return 0;
}