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// {{{ file: obr.cc | time: 23:30 13.03.2024
#define _USE_MATH_DEFINES
#include <bits/stdc++.h>

#define each(...)    for (auto& __VA_ARGS__)
#define rep(i, b, e) for (int i = (b); i <= (e); i++)
#define rev(i, b, e) for (int i = (e); i >= (b); i--)
#define mp make_pair
#define mt make_tuple
#define x first
#define y second
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define endl '\n'
#define tC template<class

using namespace std;

tC T> using V = vector<T>;
tC T1, class T2> using P = pair<T1,T2>;
tC T, class C=greater<T>> using PQ = priority_queue<T,V<T>,C>;
tC T> int sz(const T& a) { return (int)a.size(); }
tC T> bool amin(T& a, T b) { return b < a ? a = b, 1 : 0; }
tC T> bool amax(T& a, T b) { return b > a ? a = b, 1 : 0; }

using ll = long long;
using pii = P<int,int>;
using pll = P<ll,ll>;
using vi = V<int>;
using vl = V<ll>;
using vs = V<string>;
using vpi = V<pii>;
using vpl = V<pll>;

const int oo = 1e9 + 1;
const ll OO = ll(1e18) + 1;

auto now() { return chrono::high_resolution_clock::now().time_since_epoch().count(); }
mt19937 rnd(4488);
tC T> T rand(T lo, T hi) { return uniform_int_distribution<T>{lo,hi}(rnd); }

struct Debug {
#ifdef SPONGE
  tC T>Debug operator<<(const T& x) {
    cerr<<"\033[1;33m"<<x<<"\033[0m";
#else
  tC T>Debug operator<<(const T&) {
#endif
    return *this;
  }
} dbg;
#define $(x) #x<<'='<<(x)<<' '

void boost()
{
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.precision(15);
  cout.setf(ios::fixed,ios::floatfield);
  cerr.precision(15);
  cerr.setf(ios::fixed,ios::floatfield);
}

namespace { void solve(); }
// }}}

int main()
{
  boost();
  //rnd.seed(now());
  int t=1;
  //cin>>t;
  rep(i,1,t){
    //cout<<"Case #"<<i<<": ";
    solve();
  }
}

namespace {

int n,d[35];
ll odp=0;

void rek(int h,int w,int i)
{
  if(h==0 || w==0) return;

  int k=h/d[i];
  int l=w/d[i];
  if(k>0 && l>0){
    odp+=k*ll(l);
    rek(h-k*d[i],w,i-1);
    rek(k*d[i],w-l*d[i],i-1);
  }
  else{
    rek(h,w,i-1);
  }
}

void solve()
{
  int h,w;
  cin>>h>>w;

  cin>>n;
  rep(i,1,n) cin>>d[i];

  if(h%d[1]!=0 || w%d[1]!=0) cout<<-1<<endl;
  else{
    rek(h,w,n);
    cout<<odp<<endl;
  }
}

}