#include<bits/stdc++.h>
using namespace std;
typedef long double ld;
typedef long long ll;
#define rep(a, b) for(int a = 0; a < (b); ++a)
#define st first
#define nd second
#define pb push_back
#define all(a) a.begin(), a.end()
ll T[30], n;
ll solve(ll h, ll w, ll l) {
  if(l==-1) {
    if(h && w) {
      cout << -1 << '\n';
      exit(0);
    }
    return 0;
  }
  if(!h || !w) return 0;
  ll ans=(h/T[l])*(w/T[l]);
  ll x=h%T[l], y=w%T[l];
  ans+=solve(x, w-y, l-1)+solve(h-x, y, l-1)+solve(x, y, l-1);
  return ans;
}
int main() {
  ios_base::sync_with_stdio(0); cin.tie(0);
  ll h, w;
  cin >> h >> w >> n;
  rep(i, n) cin >> T[i];
  cout << solve(h, w, n-1) << '\n';
}