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#include <bits/stdc++.h>
using namespace std;

#define FOR(i, a, b) for(int i = (a); i <= (b); ++i)
#define REP(i, n) for(int i = 0; i < (n); ++i)

template<class T> int size(T && a) {
	return (int)(a.size());
}
ostream& operator << (ostream &os, string str) {
    for(char c : str) os << c;
    return os;
}
template <class A, class B> ostream& operator << (ostream &os, const pair<A, B> &p) {
	return os << '(' << p.first << "," << p.second << ')';
}
template <class T> auto operator << (ostream &os, T &&x) -> decltype(x.begin(), os) {
	os << '{';
	for(auto it = x.begin(); it != x.end(); ++it)
		os << *it << (it == prev(x.end()) ? "" : " ");
	return os << '}';
}
template <class T> ostream& operator << (ostream &os, vector<vector<T>> vec) {
    for(auto x : vec)
    	os << "\n  " << x;
    return os;
}
void dump() {}
template <class T, class... Args> void dump(T &&x, Args... args) {
    cerr << x << "; ";
    dump(args...);
}
#ifdef DEBUG
# define debug(x...) cerr << "[" #x "]: ", dump(x), cerr << '\n'
#else 
# define debug(...) 0
#endif

//--------------------------------------------------

const int INF = int(1e9) + 5;

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

	int h, w; cin >> h >> w;

	int n; cin >> n;
	vector<int> d(n);
	REP (i, n) cin >> d[i];
	d.emplace_back(INF);

	if (h % d[0] != 0 or w % d[0] != 0) {
		cout << "-1\n";
		return 0;
	}

	long long answer = 0;
	REP (i, n) {
		int a = (h % d[i + 1]);
		int b = (w % d[i + 1]);

		assert(a % d[i] == 0 and b % d[i] == 0);

		a /= d[i];
		b /= d[i];

		answer += 1LL * a * w / d[i] + 1LL * b * h / d[i] - 1LL * a * b;

		h -= a * d[i];
		w -= b * d[i];
	}

	cout << answer << '\n';
}