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#include<bits/stdc++.h>
using namespace std;
using ll = long long;
ll obr[32];
void sol(ll h, ll w, ll pos, ll &odp) {
  if(h <= 0 || w <= 0 || !pos) return;

  while(pos >= 1 && (!(h/obr[pos]) || !(w/obr[pos]))) pos--;
  ll h1 = h/obr[pos], w1 = w/obr[pos];
  if(!h1 || !w1) return;
  odp += h1*w1;
  sol(h-h1*obr[pos], w1*obr[pos], pos, odp);
  sol(h, w-w1*obr[pos], pos, odp);
}

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  ll n, h, w;
  cin >> h >> w;
  cin >> n;
  for(ll i = 1; i <= n; ++i) cin >> obr[i];
  if(h%obr[1] || w%obr[1]) {
    cout << -1;
    return 0;
  }
  ll odp = 0;
  sol(h, w, n, odp);
  cout << odp;
}