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// #pragma GCC optimize("O3")
// #pragma GCC optimize("Ofast")
// #pragma GCC optimize("unroll-loops")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
#include <bits/stdc++.h>
using namespace std;

#define PB emplace_back
#define int long long
#define ll long long
#define vi vector<int>
#define siz(a) ((int) ((a).size()))
#define rep(i, a, b) for (int i = (a); i <= (b); ++i)
#define per(i, a, b) for (int i = (a); i >= (b); --i)
void print(vi n) { rep(i, 0, siz(n) - 1) cerr << n[i] << " \n"[i == siz(n) - 1]; }

int a, b, c, s[31];

int calc(int n, int m) {
	if(!n || !m) return 0;
	if(n > m) swap(n, m);
	int x = 0;
	rep(i, 1, c) if(n >= s[i]) x = s[i];
	if(n % x == 0 && m % x == 0) return n / x * m / x;
	return m / x * calc(n, x) + calc(n, m % x);
}

signed main() {
	// freopen(".in", "r", stdin);
	// freopen(".out", "w", stdout);
	ios::sync_with_stdio(0);
	cin.tie(0), cout.tie(0);
	cin >> a >> b >> c;
	rep(i, 1, c) cin >> s[i];
	sort(s + 1, s + c + 1);
	if(a % s[1] || b % s[1]) return cout << -1, 0;
	a /= s[1], b /= s[1];
	per(i, c, 1) s[i] /= s[1];
	cout << calc(a, b);
	return cerr << endl << 1.0 * clock() / CLOCKS_PER_SEC << endl, 0;
}