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#include<bits/stdc++.h>
using namespace std;
using LL=long long;
#define FOR(i,l,r)for(int i=(l);i<=(r);++i)
#define REP(i,n)FOR(i,0,(n)-1)
#define ssize(x)int(x.size())
#ifdef DEBUG
auto operator<<(auto&o,auto x)->decltype(x.end(),o);
auto&operator<<(auto&o,pair<auto,auto>p){return o<<"("<<p.first<<", "<<p.second<<")";}
auto&operator<<(auto&o,tuple<auto,auto,auto>t){return o<<"("<<get<0>(t)<<", "<<get<1>(t)<<", "<<get<2>(t)<<")";}
auto&operator<<(auto&o,tuple<auto,auto,auto,auto>t){return o<<"("<<get<0>(t)<<", "<<get<1>(t)<<", "<<get<2>(t)<<", "<<get<3>(t)<<")";}
auto operator<<(auto&o,auto x)->decltype(x.end(),o){o<<"{";int i=0;for(auto e:x)o<<","+!i++<<e;return o<<"}";}
#define debug(X...)cerr<<"["#X"]: ",[](auto...$){((cerr<<$<<"; "),...)<<endl;}(X)
#else
#define debug(...){}
#endif

int main() {
	cin.tie(0)->sync_with_stdio(0);

	int h, w;
	cin >> h >> w;
	int n;
	cin >> n;
	vector<int> v(n);
	REP(i, n)
		cin >> v[i];
	debug(h, w, n, v);
	if (h % v[0] or w % v[0]) {
		cout << -1 << '\n';
		return 0;
	}
	function<LL(int, int, int)> calc = [&](int a, int b, int id) -> LL {
		if (a == 0 or b == 0)
			return 0;
		assert(id >= 0);
		const int d = v[id];
		const int ta = a / d;
		const int tb = b / d;
		LL ret = LL(ta) * tb;
		if (min(ta, tb) == 0) {
			ret = calc(a, b, id - 1);
		}
		else {
			ret += calc(a - ta * d, b, id - 1);
			ret += calc(ta * d, b - tb * d, id - 1);
		}
		debug(a, b, id, ret);
		return ret;
	};
	auto ans = calc(h, w, n - 1);
	cout << ans << '\n';
}