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#include <bits/stdc++.h>
#define fi first
#define sc second
#define pb push_back
#define rep(i,p,k) for(int i=(p);i<(k);++i)
#define forn(i,p,k) for(int i=(p);i<=(k);++i)
#define forr(i,k,p) for(int i=(k)-1;i>=(p);--i)
#define each(a,b) for(auto&a:b)
#define all(v)  begin(v), end(v)
#define RET(smth)   return void(cout<<(smth)<<'\n');
#define sz(v) (int)v.size()
using namespace std;
using pii = pair<int,int>;
using ll = long long;
using Vi = vector<int>;
#ifdef DEBUG
#include "debug.h"
#else
#define debug(...) ;
#endif

int main() {
#ifndef DEBUG
    cin.tie(0) -> sync_with_stdio(0);
#endif
    ll res = 0;
    vector<pii> S;
    int n;
    Vi T;
    {int w,h; cin >> w >> h;
    S.push_back({w,h});
    cin >> n;
    T.resize(n);
    for(auto&v:T) cin >> v;
    if(w % T[0] != 0 || h % T[0] != 0) return cout << "-1\n", 0;
    }
    while(!S.empty()) {
        auto [w,h] = S.back();
        S.pop_back();
        if(w == 0 || h == 0) continue;
        auto x = *prev(upper_bound(all(T),min(w,h)));
        int wr = w % x;
        int hr = h % x;
        res += 1LL*(w/x)*(h/x);
        S.emplace_back(wr,h);
        S.emplace_back(w-wr,hr);
    }
    cout << res << '\n';
    return 0;
}