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#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
 
using namespace std;
using namespace __gnu_pbds;
 
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using str = string;
 
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> oset;
 
void __print(int x) {cerr << x;}
void __print(long x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x) {cerr << x;}
void __print(unsigned long long x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
 
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ", "; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? ", " : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifdef LOCAL
#define debug(x...) cerr << "\e[91m"<<__func__<<":"<<__LINE__<<" [" << #x << "] = ["; _print(x); cerr << "\e[39m" << endl;
#else
#define debug(x...)
#endif
 
const ll mod = 1e9+7;
const int infi  = 1e9+1;
const ld eps = 1e-10L;
 
int main() {
	ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
	ll h,w; cin >> h >> w;
	ll powierzchnia = h * w;
	ll n; cin >> n;
	vector<ll> rozmiary(n+1);
	for(ll i=0;i<n;i++){
		cin >> rozmiary[i];
	}
	ll wynik = 0;
	bool mozliwe = true;
	if(powierzchnia%(rozmiary[0] * rozmiary[0]) != 0 || w%rozmiary[0] != 0 || h%rozmiary[0]!=0) mozliwe = false;
	for(ll i=n-1;i>=0;i--){
		ll tempw = w;
		if(h / rozmiary[i] == 0) continue;
		for(ll j=n-1;j>=0;j--){
			ll temph = h/rozmiary[i] * rozmiary[i];
			ll elo = (temph / rozmiary[j]) * (tempw / rozmiary[j]);
			wynik += elo;
			if(elo != 0) tempw %= rozmiary[j];
		}
		if(wynik > 0) h %= rozmiary[i];
	}
	cout << (mozliwe ? wynik : -1) << '\n';
	return 0;
}