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#include <cstdio>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <unordered_map>
#include <unordered_set>
#include <algorithm>
using namespace std;

const int N = 30 + 9;
int nextInt() { int n; scanf("%d", &n); return n; }

int a[N];

long long solve() {
	int x = nextInt();
	int y = nextInt();
	int n = nextInt();
	for (int i = 0; i < n; ++i)
		a[i] = nextInt();
	if (x % a[0] != 0) return -1;
	if (y % a[0] != 0) return -1;
	long long res = 0;

	int idx = 0;
	while (x > 0 && y > 0) {
		// assert( x % a[idx] == 0 && y % a[idx] == 0 );
		if (idx == n - 1) {
			res += 1LL * (x / a[idx]) * (y / a[idx]);
			break;
		}
		int xx = x % a[idx + 1];
		int yy = y % a[idx + 1];
		if (xx != 0) {
			res += 1LL * (xx / a[idx]) * (y / a[idx]);
			x -= xx;
		}
		if (yy != 0) {
			res += 1LL * (yy / a[idx]) * (x / a[idx]);
			y -= yy;
		}
		++idx;
	}

	return res;
}

int main() {
	long long res = solve();
	printf("%lld\n", res);
	return 0;
}