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#include<bits/stdc++.h>
#define FOR(i,a,b) for(int i=a;i<(int)b;++i)
#define FORD(i,a,b) for(int i=a;i>=(int)b;--i)
#define PB push_back
#define EB emplace_back
#define FI first
#define SE second
#define umap unordered_map
#define uset unordered_set
#define vi vector<int>
#define vvi vector<vi>
#define vll vector<ll>
#define vvll vector<vll>
#define vpii vector<pii>
#define pii pair<int, int>
#define pll pair<ll, ll>
#define ALL(X) (X).begin(),(X).end()
#ifndef DEBUG
#define endl (char)10
#endif
using namespace std;
using ll = long long;
using ld = long double;

template <class T>
istream& operator>> (istream& is, vector<T>& vec){
    FOR(i,0,vec.size()) is >> vec[i];
    return is;
}
template <class T>
ostream& operator<< (ostream& os, vector<T>& vec){
    for(auto& t : vec) os << t << " ";
    return os;
}
template<class T, class U>
ostream& operator<< (ostream& os, const pair<T, U>& p){
    os << p.FI << " " << p.SE;
    return os;
}
template<class T, class U>
istream& operator>> (istream& is, pair<T, U>& p){
    is >> p.FI >> p.SE;
    return is;
}

int main () {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    int h, w, n;
    cin >> h >> w >> n;
    vi V(n);
    cin >> V;
    if (h % V[0] != 0 || w % V[0] != 0){
        cout << -1 << endl;
        return 0;
    }
    FOR(i,1,n) V[i] /= V[0];
    h /= V[0];
    w /= V[0];
    V[0] = 1;
    ll ans = 0;
    vpii S{{w, h}};
    vpii S2;
    while(!V.empty()){
        S2.clear();
        for(auto& p : S){
            if (V.back() <= min(p.FI, p.SE));
            ll xf = p.FI / V.back();
            ll yf = p.SE / V.back();
            ans += xf * yf;
            int xc = xf * V.back();
            int yc = yf * V.back();
            if (xc < p.FI) S2.EB(p.FI - xc, p.SE);
            if (yc < p.SE) S2.EB(xc, p.SE - yc);
        }
        swap(S, S2);
        V.pop_back();
    }
    cout << ans << endl;
}