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#include <iostream>
#include <vector>
#include <algorithm>

using ll = long long;

int main() {
	std::ios_base::sync_with_stdio(0); std::cin.tie(0); std::cout.tie(0);
	ll h, w;
	std::cin >> h >> w;
	int n;
	std::cin >> n;
	std::vector<ll> D(n+1);
	D[0] = 1;
	for (int i = 1; i <= n; ++i) {
		std::cin >> D[i];
	}
	
	if (w % D[1] != 0 || h % D[1] != 0) {
		std::cout << "-1\n";
		return 0;
	}
	
	std::vector<ll> DD(n);
	for (int i = 0; i < n; ++i) {
		DD[i] = D[i+1] / D[i];
	}
	
//	std::cout << "DD:";
//	for (ll x : DD) std::cout << ' ' << x;
//	std::cout << '\n';
	
	ll ans = 0;
	for (int i = 0; i < n; ++i) {
		ll dw = w - w / DD[i] * DD[i];
		ll dh = h - h / DD[i] * DD[i];
//		std::cout << "w="<<w<<", h="<<h<<", dw=" << dw << ", dh=" << dh << '\n';
		ans += w * dh + h * dw - dw * dh;
		w /= DD[i];
		h /= DD[i];
	}
	
	ans += w * h;
	
	std::cout << ans << '\n';
}