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// Author: Kajetan Ramsza
#include "bits/stdc++.h"
using namespace std;

#define rep(i, a, b) for(int i = (a); i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

template<typename F, typename S> ostream& operator<<(ostream& os, const pair<F, S> &p) { return os<<"("<<p.first<<", "<<p.second<<")"; }
template<typename T> ostream &operator<<(ostream & os, const vector<T> &v) { os << "{"; typename vector< T > :: const_iterator it;
    for( it = v.begin(); it != v.end(); it++ ) { if( it != v.begin() ) os << ", "; os << *it; } return os << "}"; }

void dbg_out() { cerr<<'\n'; }
template<typename Head, typename... Tail> void dbg_out(Head H, Tail... T) { cerr<<' '<<H; dbg_out(T...); }

#ifdef DEBUG
#define dbg(...) cerr<<"(" << #__VA_ARGS__ <<"):", dbg_out(__VA_ARGS__)
#else
#define dbg(...) 
#endif

ll h, w;

int n;
vector<ll> vec;

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);

	cin>>h>>w;
	cin>>n;
	vec.resize(n);
	rep(i,0,n) cin>>vec[i];
	if(h % vec[0] != 0 || w % vec[0] != 0) {
		cout<<-1<<'\n';
		return 0;
	}

	ll a = h, b = w;
	ll res = 0;
	for(int i=n-1;i>=0;i--) {
		ll a1 = a / vec[i];
		ll b1 = b / vec[i];
		dbg(a,b, a1, b1, res);
		res += a1 * b1 + a1 * (w - b) / vec[i] + b1 * (h - a) / vec[i];
		a -= a1 * vec[i];
		b -= b1 * vec[i];	
	}
	cout<<res<<'\n';
}