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#include <bits/stdc++.h>
#define ll long long
#define fast ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define pb push_back
#define f first
#define s second
#define pii pair<int, int>
#define pll pair<ll, ll>
using namespace std;
ll n,w,h,kwadraty[31];
int main(){
    fast
    cin >> w >> h >> n;
    for(int i = 1;i <= n;i++)
        cin >> kwadraty[i];
    sort(kwadraty+1,kwadraty+n);
    if(w % kwadraty[1] != 0 || h % kwadraty[1] != 0){
        cout << -1 << "\n";
        return 0;
    }

    ll res = 0;
    for(int i = n;i >= 1;i--){
        if(w < kwadraty[i] || h < kwadraty[i])
            continue;
        res += (w/kwadraty[i]) * (h/kwadraty[i]);
        ll temp_h = h % kwadraty[i],temp_w = kwadraty[i] * (w/kwadraty[i]), j = i-1;
        while(temp_h){
            res += (temp_w/kwadraty[j]) * (temp_h/kwadraty[j]);
            temp_h %= kwadraty[j];
            j--;
        }
        w %= kwadraty[i];
    }
    cout << res << "\n";
}