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#pragma GCC optimize("Ofast")
//#pragma GCC target ("avx2")
//#pragma GCC optimization ("O3")
#pragma GCC optimization ("unroll-loops")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include<bits/stdc++.h>
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
//using namespace __gnu_pbds;
using namespace std;
//typedef tree<pair<int,int>,null_type,less<pair<int,int>>,rb_tree_tag,tree_order_statistics_node_update>ordered_set;
#define ll long long
#define ull unsigned long long int
#define pb push_back
#define mp make_pair
#define vi vector<int>
#define pii pair<int,int>
#define pss pair<short,short>
#define pld pair<long double,long double >
#define ld long double
#define piii  pair<pii,int>
#define vii vector<pair<int,int> >
#define st first
#define nd second
#define pll pair<ll,ll>
#define speed ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define M_PI 3.14159265358979323846
#define int long long
//const int mod=1000000007;
//const int mod=1009;
const int mod=998244353;
const int inf=1000000009;
const long long INF=1000000000000000009;
const long long big=1000000000000000;
const long double eps=0.000000000000000000001;
const int N=35;
int T[N];
int n;
pii nowy;
int oblicz(int h,int w){
    int res=0;
    pii bok;
    for(int i=n;i>=1;i--){
        if(h>=T[i]&&w>=T[i]){
            bok=mp((w/T[i])*T[i],(h/T[i])*T[i]);
            res+=(h/T[i])*(w/T[i]);
            //cout<<"o zes "<<i<<" "<<h<<" "<<w<<" "<<T[i]<<" "<<res<<"\n";
            h=h%T[i],w=w%T[i];
            break;
        }
    }
    //cout<<"siema "<<res<<"\n";
    nowy=mp(h,w);
    for(int i=n;i>=1;i--){
        if(T[i]<=h){
            res+=(h/T[i])*(bok.st/T[i]);
            h=h%T[i];
        }
        if(T[i]<=w){
            res+=(w/T[i])*(bok.nd/T[i]);
            w=w%T[i];
        }
        //cout<<i<<" elo "<<res<<"\n";
    }
    return res;
}
void solve()
{
    int h,w;
    cin>>h>>w>>n;
    for(int i=1;i<=n;i++){
        cin>>T[i];
    }
    if(h%T[1]!=0||w%T[1]!=0){
        cout<<"-1";
        return;
    }
    int wynik=0;
    nowy=mp(h,w);
    while(nowy.st&&nowy.nd){
        //cout<<nowy.st<<" "<<nowy.nd<<"\n";
        wynik+=oblicz(nowy.st,nowy.nd);
        //cout<<wynik<<" "<<nowy.st<<" "<<nowy.nd<<"\n";
    }
    cout<<wynik;
}
    
int32_t main(){
    speed
    int t = 1;
    //cin >> t;
    while(t--){
        solve();
    }
    return 0;
}