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#include<bits/stdc++.h>

using namespace std;

typedef long long ll;

int main() {
    ll w,h;
    cin>>w>>h;
    ll n;
    cin>>n;
    vector <ll> v(n);
    for(ll i=0;i<n;i++) {
        cin>>v[i];
    }
    if(w%v[0]!=0||h%v[0]!=0) {
        cout<<"-1"<<"\n";
    }
    else {
        ll res=(w*h)/(v[0]*v[0]);
        for(int i=1;i<n;i++) {
            ll a=w/v[i]*v[i];
            ll b=h/v[i]*v[i];
            if(a!=0&&b!=0) {
                res-=(a*b)/(v[i-1]*v[i-1]);
                res+=(a*b)/(v[i]*v[i]);
            }
        }
        cout<<res<<"\n";
    }
    return 0;
}