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#include<bits/stdc++.h>
using namespace std;
#ifdef DEBUG
auto&operator <<(auto& o, pair<auto, auto> p) {return o<<"("<<p.first<<", "<<p.second<<")";}
auto operator <<(auto& o, auto x)->decltype(x.end(), o) {o<<"{"; for(auto v : x) o<<v<<", "; return o<<"}";}
#define debug(X) cout<<"["#X"]"<<X<<endl;
#else
#define debug(X) {}
#endif
#define int long long
int32_t main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	int h, w;
	cin>>h>>w;
	int mm = h*w;
	int n;
	cin>>n;
	int result = 0;
	int pop;
	cin>>pop;
	if(h%pop != 0 || w%pop != 0) {cout<<-1; return 0;}
	for(int i=1;i<n;i++)
	{
		int d;
		cin>>d;
		result += ((h%d)*w + (w%d)*h - (h%d)*(w%d))/(pop*pop);
		pop = d;
		h -= h%d;
		w -= w%d;
	}
	result += (w*h)/(pop*pop);
	cout<<result;
}