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#include <cstdio>
#include <iostream>
#include <algorithm>
#include <cstring>
#include <vector>
#include <set>
#include <map>
#include <array>
#include <random>
#include <cmath>
#include <chrono>
#include <list>
#include <ctime>
#include <sstream>
#include <queue>
#include <climits>
#include <stack>
#include <valarray>
#include <random>
#include <bitset>
#include <numeric>
#include <iomanip>
#include <cassert>
using namespace std;
typedef vector<int> vi;
typedef pair<int,int> pii;
typedef long long ll;
#define rep(x, b, e) for(int x=(b); x<(e); ++x)
#define trav(a, x) for(auto& a : x)
#define ford(x, b, e) for(int x=((int)(b))-1; x>=(e); --x)
#define all(c) c.begin(),c.end()
#define sz(x) ((int)((x).size()))
#define pb push_back
#define eb emplace_back
#define st first
#define nd second
#define mp(x,y) make_pair(x,y)
typedef short int sint;
template<typename T> bool ckmin(T& a, const T& b){return b<a?a=b,1:0;}
template<typename T> bool ckmax(T& a, const T& b){return b>a?a=b,1:0;}


template <typename A, typename B>
string to_string(pair<A, B> p);

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);

string to_string(const string& s) {
  return '"' + s + '"';
}

string to_string(char c) {
  return string(1, c);
}

string to_string(const char* s) {
  return to_string((string) s);
}

string to_string(bool b) {
  return (b ? "true" : "false");
}

string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}

template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}

template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}

template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";
}

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif


// #include <atcoder/modint>
// using namespace atcoder;
// using mint = modint998244353; // modint1000000007;
// typedef vector<mint> vmi;

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count()); // use rng() to get unsigned int
// mt19937_64 for random long longs

void solve() {
	int h, w;
  cin >> h >> w;
  int n;
  cin >> n;
  vi d(n);
  rep(i, 0, n) cin >> d[i];
  ll res = 0;
  if (d[0] > 1) {
    if (h % d[0] != 0 || w % d[0] != 0) {
      cout << -1 << endl;
      return;
    }
    rep(i, 1, n) d[i] /= d[0];
    h /= d[0];
    w /= d[0];
    d[0] = 1;
  }
  rep(i, 1, n) {
    // d[i-1] = 1, 
    int c = d[i];
    ll hr = h % c;
    ll wr = w % c;
    res += hr * w + wr *h - wr * hr;
    rep(j, i, n) {
      d[j] /= c;
    }
    h /= c;
    w /= c;
  }
  res += (ll)h * w;
  cout << res << endl;
}

int main() {
	ios_base::sync_with_stdio(false); cin.tie(0);
	int t;
	// cin >> t;
	t = 1;
	while (t--) {
		solve();
	}
}