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#include<bits/stdc++.h>
using namespace std;
long long n, h, w, wyn, s,a,b, c, d, e, f;
vector<int> v;
int main(){
    cin>>h>>w>>n;
    for(int i=0; i<n; i++){
        cin>>a;
        v.push_back(a);
    }
    sort(v.begin(), v.end());
    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){
        c=(v[i-1]);
        d=(v[i]);
        s=(d*d)/(c*c);
        //cout<<s<<endl;
        e=(h/v[i]);
        f=(w/v[i]);
        wyn-=((e*f)*(s-1));
    }
    cout<<wyn;
}