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#include <algorithm>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <vector>

using namespace std;

#define PB push_back
#define MP make_pair
#define F first
#define S second
#define ALL(x) (x).begin(),(x).end()
#define SIZE(x) (int)(x).size()
#define CLEAR(tab) memset(tab, 0, sizeof(tab))
#define REP(x, n) for(int x = 0; x < (n); x++)
#define FOR(x, b, e) for(int x = (b); x <= (e); x++)
#define FORD(x, b, e) for(int x = (b); x >= (e); x--)
#define VAR(v, n) __typeof(n) v = (n)
#define FOREACH(i, c) for(VAR(i, (c).begin()); i != (c).end(); ++i)
#define DEBUG(fmt, ...) fprintf(stderr, fmt, ##__VA_ARGS__)

typedef long long int LL;
typedef pair<int,int> PII;
typedef vector<int> VI;

const int MXN = 1000000;
const int C = 262144;
const int INF = 1000000001;
const int MOD = 1000000007;

int w, h, n;
int a[MXN];
map<PII, LL> D;

LL calc(int x, int y, int ind) {
	if(x == 0 || y == 0)
		return 0;
	if(D.find(MP(x, y)) != D.end())
		return D[MP(x, y)];
	if(ind == 0 && x > 0 && y > 0) {
		return D[MP(x, y)] = -1;
	}
	int q = a[ind];
	int kx = x / q;
	int ky = y / q;
	if(kx == 0 || ky == 0) {
		return calc(x, y, ind - 1);
	}
	LL res = (LL)kx * ky;
	LL wyn1 = calc(x, y - (LL)ky * q, ind - 1);
	LL wyn2 = calc(x - (LL)kx * q, ky * q, ind - 1);
	if(wyn1 == -1 || wyn2 == -1)
		return D[MP(x, y)] = -1;
	return D[MP(x, y)] = res + wyn1 + wyn2;
}

void test() {
	scanf("%d %d", &w, &h);
	int n;
	scanf("%d", &n);
	FOR(i, 1, n)
		scanf("%d", &a[i]);
	printf("%lld\n", calc(w, h, n));
}

int main() {
	int te = 1;
//	scanf("%d", &te);
	while(te--)
		test();
	return 0;
}