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#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define mid ((l+r)/2)
#define siz(x) (int)(x).size()
#define BOOST ios_base::sync_with_stdio(0), cin.tie(0)
#define deb(x) cout << #x << ": " << x << "\n"
typedef long long ll;
typedef long double ld;
typedef pair<int, int> ii;

int main(){
	BOOST;	
	int h, w; cin >> h >> w;
	int n; cin >> n;
	vector<int> t(n);
	for(int i=0; i<n; i++){
		int x; cin >> x;
		t.pb(x);
	}
	reverse(all(t));
	ll ans = 0;
	int a = 0, b = 0;
	for(int i=0; i<n; i++){
		ans += (ll)(b/t[i])*((h-a)/t[i]);
		a += (h-a)/t[i]*t[i];
		ans += (ll)(a/t[i])*((w-b)/t[i]);
		b += (w-b)/t[i]*t[i];
	}
	if(a != h || b != w){
		cout << -1 << "\n";
	}
	else{
		cout << ans << "\n";
	}
}