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// clang-format off
#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define ssize(x) int(x.size())
template<class A,class B>auto&operator<<(ostream&o,pair<A,B>p){return o<<'('<<p.first<<", "<<p.second<<')';}
template<typename T,typename = typename enable_if<!is_same<T,string>::value>::type>
auto operator<<(ostream&o,T x)->decltype(x.end(),o){o<<'{';int i=0;for(auto e:x)o<<(", ")+2*!i++<<e;return o<<'}';}
#ifdef DEBUG
#define debug(x...) cerr<<"["#x"]: ",[](auto...$){((cerr<<$<<"; "),...);}(x),cerr<<'\n'
#else
#define debug(...) {}
#endif
// clang-format on

ll d[32];

int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    ll w, h, n, ans = 0, a, b;
    cin >> h >> w >> n;
    for (int i = 1; i <= n; i++)
        cin >> d[i];

    if (h % d[1] || w % d[1])
    {
        cout << -1 << '\n';
        return 0;
    }

    for (int i = 1; i < n; i++)
    {
        a = h % d[i + 1] / d[i];
        b = w % d[i + 1] / d[i];
        ans += a * w / d[i] + b * h / d[i] - a * b;
        h -= a * d[i];
        w -= b * d[i];
    }

    ans += w * h / d[n] / d[n];

    cout << ans << '\n';

    return 0;
}