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#include <bits/stdc++.h>
using namespace std;
#define ll long long
ll x, y, N, ans;
vector<ll> v(31);
int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);

    cin >> x >> y >> N;

    for (int i=0; i<N; i++) cin >> v[i];

    if (x%v[0]!=0 || y%v[0]!=0){
        cout << "-1";
        return 0;
    }

    for (int i=0; i<N-1; i++){
        ll z=x%v[i+1];
        x-=z;
        z/=v[i];
        ans+=(z*(y/v[i]));
        z=y%v[i+1];
        y-=z;
        z/=v[i];
        ans+=(z*(x/v[i]));
    }
    ans+=((x/v[N-1])*(y/v[N-1]));
    cout << ans;

    return 0;
}