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#include <bits/stdc++.h>

#define ndl '\n'
#define ll long long
#define INF 1000000007
#define st first
#define nd second
#define debug(x) cout << #x << ": " << x << ndl
#define pb push_back
#define pob pop_back
#define pf push_front
#define pof pop_front
#define lb lower_bound
#define ub upper_bound
#define all(x) (x).begin(), (x).end()

using namespace std;

ll minimalizuj(ll a, ll b, vector<ll> &t) {
    for(ll i = t.size()-1; i>=0; i--) 
        if(t[i] <= a && t[i] <= b) {
            ll ile_a = a/t[i], ile_b = b/t[i];
            return ile_a * ile_b + minimalizuj(a-ile_a*t[i], b-ile_b*t[i], t) + minimalizuj(ile_a*t[i], b-ile_b*t[i], t) + minimalizuj(a-ile_a*t[i], ile_b*t[i], t);
        }

    return 0;
}

int main(){
    ios_base::sync_with_stdio(0); cin.tie(0);
    ll a, b, n;
    cin>>a>>b>>n;
    vector<ll> t(n);
    for(auto &x:t) cin>>x;
    if(a%t[0] || b%t[0]) {
        cout<<-1;
        return 0;
    }

    cout<<minimalizuj(a, b, t);
}