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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,pair<auto,auto>p){return o<<"("<<p.first<<", "<<p.second<<")";}
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> t(n);
	REP (i, n)
		cin >> t[i];
	sort(t.rbegin(), t.rend());
	function<LL(int, int, int)> rek = [&](int a, int b, int p) {
		if (p == n)
			return 0ll;
		if (a < t[p] || b < t[p])
			return rek(a, b, p + 1);
		LL ca = a / t[p];
		LL cb = b / t[p];
		int na = a % t[p];
		int nb = b % t[p];
		return ca * cb +
			rek(na, b, p + 1) +
			rek(a - na, nb, p + 1);
	};
	if (w % t.back() || h % t.back()) {
		cout << -1 << endl;
		return 0;
	}
	cout << rek(w, h, 0) << endl;
}