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#include <bits/stdc++.h>
using namespace std;
typedef vector<int> VI;
typedef pair <int,int> ii;
typedef long long LL;
#define pb push_back
const int INF = 2147483647;
const int N = 3005;

int h, w, n, i, tab[33];

LL get(int h, int w, int ind) {
	if (tab[ind] > h || tab[ind] > w) return get(h, w, ind - 1);
	LL res = (h / tab[ind]) * 1LL * (w / tab[ind]);
	if (h % tab[ind] != 0) res += get(h % tab[ind], w, ind - 1);
	if (w % tab[ind] != 0) res += get(h - h % tab[ind], w % tab[ind], ind - 1);
	return res;
}

int main() {
scanf("%d %d %d", &h, &w, &n);
for (i=0;i<n;i++) scanf("%d", &tab[i]);
if (h % tab[0] != 0 || w % tab[0] != 0) {
	printf("-1\n");
	return 0;
}
printf("%lld\n", get(h, w, n - 1));
return 0;
}