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/*
 * Opis: Główny nagłówek
 */
#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);
	LL h, w;
	cin >> h >> w;
	int k;
	cin >> k;
	vector<LL> divs(k);
	for (auto &d : divs) {
		cin >> d;
	}
	if (h % divs[0] != 0 || w % divs[0] != 0) {
		cout << -1 << "\n";
		return 0;
	}

	reverse(divs.begin(), divs.end());
	LL taken = 0;
	LL res = 0;
	for (auto d : divs) {
		LL fits = (h / d) * (w / d);
		res += fits - taken/d/d;
		taken = fits * d * d;
	}
	cout << res << endl;
}