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#include <bits/stdc++.h>
using namespace std;
#define int long long
constexpr int maxN = 50+7;

int h,w,n,a[maxN];

int pokryj(int x, int y, int val){
	return (x/val) * (y/val);
}

int32_t main(){
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	
	cin>>h>>w;
	if(h < w)
		swap(h,w);
	cin>>n;
	for(int i=1;i<=n;i++)
		cin>>a[i];
	if(h % a[1] != 0 || w % a[1] != 0){
		cout<<-1<<"\n";
		return 0;
	}
	int lh = 0, lw = 0, ans = 0;
	a[n+1] = h+1;
	for(int i=n;i>=1;i--){
		if(a[i] > min(h,w))
			continue;
		int ch = h / a[i] * a[i];
		int cw = w / a[i] * a[i];
		int diffh = ch - lh;
		int diffw = cw - lw;
		//cout<<diffh<<' '<<diffw<<"to jest to\n";
		//cout<<ch<<' '<<cw<<"\n";
		ans += pokryj(diffh, cw, a[i]);
		if(a[i+1] <= min(h,w))
			ans += pokryj(diffw, ch, a[i]);
		if(a[i+1] <= min(h,w) && diffh && diffw)
			ans -= pokryj(diffh, diffw, a[i]);
		lh = h / a[i] * a[i];
		lw = w / a[i] * a[i];
		//cout<<ans<<" to jest ans\n";
	}
	cout<<ans<<"\n";
}
/*

6 10
3
1 3 6

7 10
3
1 3 6

*/