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#include <bits/stdc++.h>
using namespace std;
using ll = long long;

int n;
vector<ll> vec;
ll h, w;
ll ans = 0;

ll solve(int ind){
    ll wys = vec[ind];
    ll res = 0;
    ll sz = w;
    for(int j = ind; j >= 0; j--){
        if(vec[j] <= sz){
            ll ile = (ll)((ll)wys / vec[j]) * ((ll)sz / vec[j]);
            res += ile;
            sz = (ll)sz - (ll)vec[j] * ((ll)sz / vec[j]);
        }
    }
    
    return res;
}

int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(NULL);
    cout.tie(NULL);
    
    cin >> h >> w;
    cin >> n;
    vec.resize(n);
    for(int i = 0; i < n; i++){
        cin >> vec[i];
    }
    if(h % vec[0] != 0 || w % vec[0] != 0){
        cout << -1 << '\n';
        return 0;
    }
    
    for(int i = n - 1; i >= 0; i--){
        if(vec[i] <= h){
            ll ile = (ll)h / vec[i];
            ll cntOne = solve(i);
            h = (ll)h - (ll)ile * vec[i];
            ans = (ll)ans + (ll)cntOne * ile;
        }
    }
    
    cout << ans << '\n';
    
    return 0;
}