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#include <stdio.h>
#include <time.h>
#include <math.h>
#include <stdlib.h>
#include <string.h>
#include <map> 
#include <string>
#include <vector>  
#include <iostream> 
#include <sstream> 
#include <queue>
#include <algorithm>
#include <assert.h>


using namespace std;

#define ll long long
#define PB 		push_back
#define FOR(a,start,end) 	for(int a=int(start); a<int(end); a++)
#define INF 		INT_MAX
#define SORT(a) 	sort(a.begin(),a.end()) 
#define CL(a,x) 		memset(a,x,sizeof(a))
#define REP(a,x)	for(int a=0;a<x;a++)
#define REP1(a,x)	for(int a=1;a<=x;a++)
#define MP 		make_pair



typedef vector<ll>     vi;
typedef pair<ll, ll>     pii;
typedef vector<string> vs;
typedef vector<pii> vii;
typedef vector<vector<ll> > vvi;
typedef vector<vector<string> > vvs;
typedef vector<pair<string, string> > vss;
typedef pair<string, string> pss;
typedef pair<ll, pii> ppii;
typedef vector<ppii> vppii;
typedef vector<vector<pii> > vvii;
typedef vector<vvi> vvvi;


ll  n, k, q, d, r, g, b, z, p, a, c,h,w;
char zn;
string wa;
vi vk, va, vb;

ll dodaj(ll a, ll b) {
	ll a0 = a;
	ll k = 0;
	g = 1;
	int j = 0;
	for (int i = n - 1; i >= 0; i--) {
		if (va[i] > a)
			continue;
		if (g == 1)
			g = va[i];
		j = max(j, i);
		k += (a / va[i]) * (g / va[i]);
		
		a -= (a / va[i]) * va[i];
		if (a == 0) {
			break;
		}
	}
	
	k *= (b / va[j]);
	
	if ((b - (b / va[j]) * va[j]) > 0) {
		k += dodaj(min((b - (b / va[j]) * va[j]), a0), max((b - (b / va[j]) * va[j]), a0));
	}
	return k;
}
int main() {
	//freopen("c:\\wojtek\\uva\\pa\\debug\\t1.in", "rt", stdin);
	//d = 1000000007;
	std::ios_base::sync_with_stdio(false);
	std::cin.tie(NULL);
	//	while (1) {
	cin >> h>>w;
	cin >> n;
	va.assign(n, 0);
	for (int i = 0; i < n; i++) {
		cin >> va[i];
	}
	if((w%va[0]!=0)||(h%va[0]!=0))
		cout<<-1<<'\n';
	else {
		a = min(h, w);
		b = max(h, w);
		k = dodaj(a, b);
		cout << k << '\n';
		
	}
	

	//}

	return 0;

}