//Solution by Mikołaj Kołek

#include "bits/stdc++.h"
#define intin *istream_iterator<int>(cin)

using namespace std;

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	
	int h = intin, w = intin, n = intin;
	vector<int> d(n);
	copy_n(istream_iterator<int>(cin), n, d.begin());
	sort(d.begin(), d.end());
	
	if(h % d[0] != 0 or w % d[0] != 0) {
		cout << -1;
		return 0;
	}
	
	function<long long(int, int)> check = [&] (long long curH, long long curW) {
		if(curH == 0 or curW == 0)
			return 0ll;
		
		for(int i = d.size() - 1; i >= 0; i--) {
			if(d[i] <= curH and d[i] <= curW) {
				long long result = (curH / d[i]) * (curW / d[i]);
				result += check(curH - (curH % d[i]), curW % d[i]);
				result += check(curH % d[i], curW);
				
				return result;
			}
		}
		
		return 0ll;
	};
	
	cout << check(h, w);
	
	return 0;
}