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

#define int long long

int w, h, n, ans;
int d[35];
bool wa;

int cnt(int a, int b){
	if(a > b) swap(a,b);
	int act = 0;
	//lowerbound a
	for(int i=n-1; i>=0; i--){
		if(d[i] > a) continue;
		act = d[i];
		break;
	}
	if(act == 0){
		wa = 1;
		return 0;
	}
	int cost = b/act;
	if(b % act) cost += cnt(act,b%act);
	int res = a/act * cost;
	if(a % act) res += cnt(a%act,b);
	return res;
}

signed main(){
	ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
	
	cin >> w >> h >> n;
	
	for(int i=0; i<n; i++) cin >> d[i];
	
	ans = cnt(w, h);
	
	if(!wa) cout << ans << '\n';
	else cout << -1 << '\n';
	
	return 0;
}