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#include <bits/stdc++.h>

using namespace std;

long n;
long A[30];

long long cos(long a,long b){
//  cout<<a<<" "<<b<<"\n";
  if(a==0 or b==0)return 0;
  long m=n;
  while(m){
    m--;
    if(a<A[m] or b<A[m])continue;
    break;
  }
  if(a<A[m] or b<A[m])return -1;
  long as=a%A[m];long heses=b%A[m];
//  if(as>heses)a+=as;
//  else b+=heses;
  long long p,q,r,s;
  p=cos(as,heses);
  q=cos(as,b-heses);
  r=cos(a-as,heses);
  s=(long long)(a/A[m])*(b/A[m]);
  if(p==-1 or q==-1 or r==-1)return -1;
  else return p+q+r+s;
//  return cos(as,heses)+cos(as,b-heses)+cos(a-as,heses)+(long)(a/A[m])*(b/A[m]);
}

int main(){
ios_base::sync_with_stdio(false);
cin.tie();
cout.tie();
  long h,w;cin>>h>>w;
  cin>>n;
  for(long i=0;i<n;i++)cin>>A[i];
  long long x=cos(h,w);
  if(x<0)cout<<"-1";
  else cout<<x;
  cout<<"\n";
return 0;
}