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#include <bits/stdc++.h>
#ifdef LOC
#include "debuglib.h"
#else
#define deb(...)
#define DBP(...)
#endif
using namespace std;
using   ll         = long long;
using   vi         = vector<int>;
using   pii        = pair<int, int>;
#define pb           push_back
#define mp           make_pair
#define x            first
#define y            second
#define rep(i, b, e) for (int i = (b); i < (e); i++)
#define each(a, x)   for (auto& a : (x))
#define all(x)       (x).begin(), (x).end()
#define sz(x)        int((x).size())

vector<ll> d;

ll solve(ll w, ll h) {
	if (w < h) swap(w, h);
	if (h == 0) return 0;
	ll a = 0;
	for (int i = sz(d)-1; i >= 0; i--) {
		if (d[i] <= h) {
			a = d[i];
			break;
		}
	}
	if (a == 0) return -1;
	ll right = solve(w%a, h);
	ll up = solve(w/a*a, h%a);
	if (right == -1 || up == -1) return -1;
	return (w/a) * (h/a) + right + up;
}

void run() {
	int w, h, n;
	cin >> w >> h >> n;
	d.resize(n);
	each(e, d) cin >> e;
	cout << solve(w, h) << '\n';
}

int main() {
	cin.sync_with_stdio(0); cin.tie(0);
	cout << fixed << setprecision(10);
	run();
	cout << flush; _Exit(0);
}