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#include <iostream>
#include <map>
#include <unordered_map>
#include <vector>
#include <unordered_set>
#include <set>
#include <queue>
#include <utility>
#include <algorithm>
#include <cassert>
#include <iomanip>
#include <tuple>
#include <stack>
#include <deque>

using namespace std;

///// TEMPLATES
typedef long long ll;
typedef tuple<ll, ll> ti2;
typedef tuple<ll, ll, ll> ti3;
typedef tuple<ll, ll, ll, ll> ti4;

typedef vector<bool>   vb;
typedef vector<vb>     vvb;
typedef vector<string> vs;

typedef vector<ll>  vi;
typedef vector<ti2> vi2;
typedef vector<ti3> vi3;
typedef vector<vi>  vvi;

typedef set<ll>  si;
typedef set<ti2> si2;
typedef set<ti3> si3;

typedef multiset<ll>  msi;
typedef multiset<ti2> msi2;
typedef multiset<ti3> msi3;

typedef deque<ll>  dqi;
typedef deque<ti2> dqi2;
typedef deque<ti3> dqi3;

template<typename T> using PQS = priority_queue<T, vector<T>, greater<T> >;
template<typename T> using PQG = priority_queue<T>;

///// OUT OPERATORS
ostream& operator<<(ostream& os, const ti2& x)  { os << "{ "; auto [a, b] = x;       os << a << ", " << b;                            os << " }"; return os; }
ostream& operator<<(ostream& os, const ti3& x)  { os << "{ "; auto [a, b, c] = x;    os << a << ", " << b << ", " << c;               os << " }"; return os; }
ostream& operator<<(ostream& os, const ti4& x)  { os << "{ "; auto [a, b, c, d] = x; os << a << ", " << b << ", " << c << ", " << d;  os << " }"; return os; }

ostream& operator<<(ostream& os, const vi& x)   { os << "{ "; for (auto e : x) os << e << " "; os << "}"; return os; }
ostream& operator<<(ostream& os, const vs& x)   { os << "{ "; for (auto e : x) os << e << " "; os << "}"; return os; }
ostream& operator<<(ostream& os, const vb& x)   { os << "{ "; for (auto e : x) os << e << " "; os << "}"; return os; }
ostream& operator<<(ostream& os, const vvb& x)  { os << "{ "; for (auto e : x) os << e << " "; os << "}"; return os; }
ostream& operator<<(ostream& os, const vi2& x)  { os << "{ "; for (auto e : x) os << e << " "; os << "}"; return os; }
ostream& operator<<(ostream& os, const vi3& x)  { os << "{ "; for (auto e : x) os << e << " "; os << "}"; return os; }
ostream& operator<<(ostream& os, const vvi& x)  { os << "{ "; for (auto e : x) os << e << " "; os << "}"; return os; }

ostream& operator<<(ostream& os, const msi& x)  { os << "{ "; for (auto e : x) os << e << " "; os << "}"; return os; }
ostream& operator<<(ostream& os, const msi2& x) { os << "{ "; for (auto e : x) os << e << " "; os << "}"; return os; }
ostream& operator<<(ostream& os, const msi3& x) { os << "{ "; for (auto e : x) os << e << " "; os << "}"; return os; }

ostream& operator<<(ostream& os, const dqi& x)  { os << "{ "; for (auto e : x) os << e << " "; os << "}"; return os; }
ostream& operator<<(ostream& os, const dqi2& x) { os << "{ "; for (auto e : x) os << e << " "; os << "}"; return os; }
ostream& operator<<(ostream& os, const dqi3& x) { os << "{ "; for (auto e : x) os << e << " "; os << "}"; return os; }

///// IN OPERATORS
istream& operator>>(istream& is, ti2& x) { ll a, b;       is >> a >> b;           x = {a, b}; return is; }
istream& operator>>(istream& is, ti3& x) { ll a, b, c;    is >> a >> b >> c;      x = {a, b, c}; return is; }
istream& operator>>(istream& is, ti4& x) { ll a, b, c, d; is >> a >> b >> c >> d; x = {a, b, c, d}; return is; }


int log_floor(long long x) { return 64 - __builtin_clzll(max(1LL, x)); } // PLUS ONE!
int bit_cnt(ll x) { return __builtin_popcountll(x); }

void rev(auto& v) { reverse(v.begin(), v.end()); }
void sor(auto& v) { sort(v.begin(), v.end()); }

// ⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣀⣀⣤⣀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣀⣠⣀⣀⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
// ⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⣾⣿⣿⣿⣿⣿⣿⣆⠀⢀⣀⣀⣤⣤⣤⣦⣦⣤⣤⣄⣀⣀⠀⢠⣾⣿⣿⣿⣿⣿⣷⣦⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
// ⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣰⣿⣿⣿⣿⣿⣿⣿⣿⡿⠟⠛⠉⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⠙⠛⠿⣿⣿⣿⣿⣿⣿⣿⣿⣷⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
// ⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣿⣿⣿⣿⣿⣿⣿⠟⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⠙⢿⣿⣿⣿⣿⣿⣿⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
// ⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢻⣿⣿⣿⣿⡟⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠙⣿⣿⣿⣿⣿⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
// ⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⠻⢿⣿⠟⠀⠀⠀⠀⠀⣀⣤⣤⣤⡀⠀⠀⠀⠀⠀⢀⣤⣤⣤⣄⡀⠀⠀⠀⠀⠘⣿⡿⠿⠃⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
// ⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢠⡟⠀⠀⠀⠀⣠⣾⣿⣿⣟⣿⡇⠀⠀⠀⠀⠀⢸⣿⣿⣻⣿⣿⣦⠀⠀⠀⠀⠸⣧⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
// ⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣼⠁⠀⠀⠀⠀⣿⣿⣿⣿⣿⡟⢠⣶⣾⣿⣿⣷⣤⢽⣿⣿⣿⣿⣿⡇⠀⠀⣀⣤⣿⣷⣴⣶⣦⣀⡀⠀⠀⠀⠀⠀⠀⠀
// ⠀⠀⠀⠀⠀⠀⠀⠀⢀⣠⣤⣤⣠⣇⠀⠀⠀⠀⠀⣿⣿⣿⣿⣿⠀⠘⠻⣿⣿⣿⡿⠋⠀⢹⣿⣿⣿⣿⡇⠀⣿⣿⣿⡏⢹⣿⠉⣿⣿⣿⣷⠀⠀⠀⠀⠀⠀
// ⠀⠀⠀⠀⠀⠀⢠⣾⣿⣿⣿⣿⣿⣿⣿⣶⣄⠀⠀⠹⣿⣿⠿⠋⠀⢤⣀⢀⣼⡄⠀⣠⠀⠈⠻⣿⣿⠟⠀⢸⣿⣇⣽⣿⠿⠿⠿⣿⣅⣽⣿⡇⠀⠀⠀⠀⠀
// ⠀⠀⠀⠀⠀⠀⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣆⠀⠀⠀⠀⠀⠀⠀⠀⠈⠉⠁⠉⠉⠀⠀⠀⠀⠀⠀⠀⠀⠈⣿⣿⣟⠁⠀⠀⠀⠈⣿⣿⣿⡇⠀⠀⠀⠀⢀
// ⠛⠛⠛⠛⠛⠛⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛⠛
// ⠀⠀⠀⠀⠀⠀⠘⠛⠻⢿⣿⣿⣿⣿⣿⠟⠛⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
// ⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⠀⠈⠉⠀⠀⠀⠀                          [ STASZEK ]                   
//

// inject here

void ex() {
    cout << "-1\n";
    exit(0);
}

ll divide(ll h, ll w, const vi& v, int it) {
    if (h == 0 || w == 0) return 0LL;
    if (it == v.size()) ex();

    ll h2 = h % v[it];
    ll h1 = h - h2;

    ll w2 = w % v[it];
    ll w1 = w - w2;
 
    ll res = h1 * w1 / (v[it] * v[it]);
    res += divide(h1, w2, v, it + 1);
    res += divide(h2, w, v, it + 1);
    return res;
}

void solve(){
    ll h, w, n;
    cin >> w >> h >> n;
    vi v(n);
    for (int i = 0; i < n; i ++) {
        cin >> v[i];
    }
    sor(v); rev(v);
    cout << divide(h, w, v, 0);
}

int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(nullptr);
    cout << setprecision(15) << fixed;
    int t = 1;
  //  cin >> t;
    while (t --) solve();
    return 0;
}