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#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <chrono>
#include <cmath>
#include <complex>
#include <cstring>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <vector>
#include <climits>
using namespace std;
 
using ll = long long;
using ul = unsigned long long;
using db = long double;
using pi = pair<int, int>;
using vi = vector<int>;
using vl = vector<ll>;
using vpi = vector<pi>;
#define mp make_pair
#define pb push_back
#define eb emplace_back
#define x first
#define y second

template<class T> using V = vector<T>; 
template<class T, size_t SZ> using AR = array<T,SZ>; 

#define FOR(i,a,b) for (int i = (a); i < (b); ++i)
#define F0R(i,a) FOR(i,0,a)
#define ROF(i,a,b) for (int i = (b)-1; i >= (a); --i)
#define R0F(i,a) ROF(i,0,a)
#define each(a,x) for (auto& a: x)
 
#define sz(x) int((x).size())
#define all(x) (x).begin(), (x).end()
#define rep(i,a,b) for(int i = (a); i < (b); i++)
#define per(i,a,b) for(int i = (b) - 1; i >= (a); i--)

#ifdef LOCAL 
template<class A, class B> auto& operator<<(auto &o, pair<A, B> p) { return o << '(' << p.x << ", " << p.y << ')'; }
auto& operator<<(auto& o, auto a) {
    o << "{";
    for (auto b : a) o << b << ", ";
    return o << "}";
}
void dump(auto... x) { ((cerr << x << ", "), ...) << "\n"; }
#define debug(x...) cerr << "[" #x "]: ", dump(x)
#else
#define debug(...) ;
#endif

template<class T> bool ckmin(T& a, const T& b) { return b < a ? a = b, 1 : 0; }
template<class T> bool ckmax(T& a, const T& b) { return a < b ? a = b, 1 : 0; }
template<class T> int lwb(V<T>& a, const T& b) { return int(lb(all(a),b)-bg(a)); }
template<class T> int upb(V<T>& a, const T& b) { return int(ub(all(a),b)-bg(a)); }
template<class T> void remDup(vector<T>& v) { sort(all(v)); v.erase(unique(all(v)),end(v)); }

signed main() {
    ios_base::sync_with_stdio(false); cin.tie(nullptr);

    ll h, w;
    int n;
    cin >> h >> w >> n;
    vl d(n);
    rep(i,0,n) cin >> d[i];
    if (h % d[0] != 0 || w % d[0] != 0) {
        cout << "-1\n";
        return 0;
    }

    ll res = 0;

    if (h > w) swap(h, w);

    while (h) {
        while (d.back() > h) d.pop_back();
        ll k = h / d.back();
        h -= k * d.back();
        n = sz(d);
        ll cur_w = w;
        per(i,0,n) {
            ll cnt = cur_w / d[i];
            cur_w -= d[i] * cnt;
            res += cnt * k * (d.back() / d[i]);
        }
        assert(cur_w == 0);
    }

    cout << res << '\n';

    return 0;
}