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#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
 
#define mp make_pair
#define pb push_back
#define ppb pop_back
#define pf push_front
#define ppf pop_front
#define eb emplace_back
#define st first
#define nd second
#define vt vector
#define VAR(__var)  #__var << ": " << __var << " "
#define PAIR(__var) #__var << ": " << __var.first << ", " << __var.second << " "
#define FOR(__var, __start, __end)  for(int __var=__start; __var<__end; ++__var)
#define FORB(__var, __start, __end) for(int __var=__start; __var>__end; --__var)
#define REP(__var, __cnt) for(int __var=0; __var<(__cnt); ++__var)
#define EACH(__var, __x) for(auto __var : __x)
#define maxi(__x, __y) __x = (__x>__y?__x:__y)
#define mini(__x, __y) __x = (__x<__y?__x:__y)
#define all(__var)     (__var).begin(),(__var).end()
#define rall(__var)    (__var).rbegin(),(__var).rend()
#define sz(__var)      (int)(__var).size()

using namespace std;
 
using namespace __gnu_pbds;
template <typename T>
using ord_set = tree<T,null_type,less<T>,rb_tree_tag,tree_order_statistics_node_update>;

#ifdef DEBUG
template<typename Stream, typename T1, typename T2>
Stream& operator << (Stream& out, pair<T1, T2> a) {return out << "(" << a.st << ", " << a.nd << ")";}
 
template<typename Stream, typename T>
Stream &operator << (Stream& out, T v) {
	out << "{";
	int i = 0;
	for (auto x : v)
		out << x << ((++ i) != sz(v) ? ", " : "");
	return out << "}";
}
 
template<typename... Args>
void dump(Args... x) {((cerr << x << ", "), ...) << '\n';}
 
#define debug(x...) cerr << "[" #x "]: ", dump(x)
#else
#define debug(...) 0
#endif
 
typedef       long long ll;
typedef     long double ld;
typedef   pair<ll, ll> pll;
typedef pair<int, int> pii;

const bool __test_cases = false;
 
const int INF = 1e9+2137;

ll count(ll w, ll h, vt<ll> &a) {
	if(w == 0 || h == 0)
		return 0;
	
	ll picked = -1;
	for(auto x : a) {
		if(x <= w && x <= h) {
			picked = x;
			break;
		}
	}

	if(picked == -1)
		return -1;

	ll l = (w/picked) * picked;
	ll k = (h/picked) * picked;

	ll res = (w/picked) * (h/picked);

	ll got = count(w-l, h-k, a);
	if(got == -1)
		return -1;
	res += got;

	got = count(l, h-k, a);
	if(got == -1)
		return -1;
	res += got;

	got = count(w-l, k, a);
	if(got == -1)
		return -1;
	res += got;
	
	return res;
}

void solve() {
	ll w, h;
	cin >> w >> h;

	int n;
	cin >> n;
	
	vt<ll> a(n);
	FOR(i, 0, n)
		cin >> a[i];
	
	reverse(all(a));

	cout << count(w, h, a) << '\n';
}

int32_t main() {
	ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
	
	int t=1;
	if(__test_cases)
		cin >> t;
	while(t--) {
		solve();
	}

	return 0;
}