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

bool impossible = false;

ll find(ll h, ll w, vector <ll> d){
    if(h < 1 || w < 1) return 0;
    if(d.size() == 0){
        impossible = true;
        return 0;
    }
    if(d.back() > h || d.back() > w){
        d.pop_back();
        return find(h, w, d);
    }
    ll ans = 0;
    ll x = d.back(); d.pop_back();
    ll maxh = (h/x)*x;
    ll maxw = (w/x)*x;
    ans += (maxh/x) * (maxw/x);
    ans += find(h - maxh, maxw, d);
    ans += find(maxh, w - maxw, d);
    ans += find(h - maxh, w - maxw, d);
    return ans;
}

int main(){
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    ll h, w, n; cin >> h >> w >> n; 
    vector <ll> d(n); for(auto& x:d) cin >> x;
    sort(d.begin(), d.end());
    ll ans = find(h, w, d);
    cout << (impossible ? -1 : ans);  
}