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#include <bits/stdc++.h>
using namespace std;

int main()
{
	long long h, w, n, o, a, b;
	cin >> h >> w >> n;
	
	vector<long long> obr;
	
	for(int i = 0; i < n; i++)
	{
		cin >> o;
		obr.push_back(o);
	}
	
	long long hz = h;
	long long wz = w;
	long long ans = 0;
	long long ktore = n-1;
	
	if(h%obr[0] != 0 || w%obr[0] != 0)
	{
		cout << -1 << '\n';
		return 0;
	}
	//cout << h << w << obr[0];
	
	while(obr[ktore]>h || obr[ktore]>w)
	{
		ktore--;
	}
	ans+=(h/obr[ktore])*(w/obr[ktore]);
	hz=h%obr[ktore];
	wz=w%obr[ktore];
	ktore--;
	while(hz>0 || wz>0)
	{
		a = hz/obr[ktore];
		b = wz/obr[ktore];

		ans+= a*(w-wz)/obr[ktore] + b*(h-hz)/obr[ktore] + a*b;
		
		wz-=b*obr[ktore];
		hz-=a*obr[ktore];
		
		ktore--;
	}
	cout << ans << "\n";
}