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#include <iostream>
#include <vector>
#include <set>
#include <map>
#include <algorithm>


using namespace std;

#define pb push_back
#define mp make_pair
#define st first
#define nd second
#define x first
#define y second

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<ll> vll;


int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    
    ll h, w;
    cin >> h >> w;

    int n;

    cin >> n;

    vll d;

    for(int i=0; i<n; i++){
        ll di;
        cin >> di;
        d.pb(di);
    }

    sort(d.begin(), d.end());

    ll smallest = d[0];

    if(h%smallest != 0 || w%smallest != 0){
        cout << "-1\n";
    }else{
        ll ans=(w/smallest) * (h/smallest);
    
        for(int i=1; i<d.size(); i++){
           ll c = (w/d[i]) * (h/d[i]);
           ll s = c * d[i] * d[i];
           ans -= s/d[i-1]/d[i-1];
           ans += c;
        }
        cout << ans << "\n";
    }

    

    return 0;
}