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#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef pair<int, int> pii;

#define FOR(x, b, e) for(int x = (b); x <= (e); ++(x))
#define FORD(x, b, e) for(int x = (b); x >= (e); --(x))
#define REP(x, n) for(int x = 0; x < (n); ++x)
#define ALL(c) (c).begin(), (c).end()
#define SIZE(x) ((int)(x).size())
#define FOREACH(a, b) for (auto&a : (b))
#define PB push_back
#define PF push_front
#define MP make_pair
#define ST first
#define ND second
#define DBG(vari) cerr<<#vari<<" = "<<(vari)<<endl;

template <typename T>
std::ostream& operator<<(std::ostream &output, const vector<T> &vec)
{
    output << "[";
    FOREACH(x, vec) output << x << ", ";
    output << "]";
    return output;
}

template <typename T, typename U>
std::ostream& operator<<(std::ostream &output, const pair<T,U> &p)
{
    output << "(";
    output << p.ST << ", " << p.ND;
    output << ")";
    return output;
}


int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);

  int H, W, N; cin >> H >> W >> N;
  vi D(N); REP(i, N) cin >> D[i];

  if (H % D[0] == 0 && W % D[0] == 0) {
    ll res = ll(H/D[0])*(W/D[0]);
    FOR(i, 1, N-1) {
      ll ile = ll(H/D[i])*(W/D[i]);
      res += ile - ile * (D[i]/D[i-1])*(D[i]/D[i-1]);
    }
    cout << res; 
  }
  else
  {
    cout << "-1";
  }

}