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#include <bits/stdc++.h>

#define ll long long
#define fors(u, n, s) for(ll u = (s); u < (n); u++)
#define foru(u, n) fors(u, n, 0)
#define vec vector
#define pb push_back
#define f first
#define s second
#define ir(a, b, x) (((a) <= (x)) && ((x) <= (b)))
#define pint pair<ll, ll>

using namespace std;

const int N = 30;

ll h;
ll w;
ll n;

ll d[N];

ll solve_ideal(ll idx, ll y){
	ll out = 0;
	ll idx_ = idx;
	while(idx_ >= 0){
		ll y_cnt = y/d[idx_];
		y -= y_cnt*d[idx_];
		out += y_cnt*d[idx]/d[idx_];
		idx_ --;
	}
	return out;
}

ll solve(ll x, ll y){
	ll out = 0;
	ll idx = n-1;
	while(idx >= 0){
		ll cnt = x/d[idx];
		x -= cnt*d[idx];
		out += cnt*solve_ideal(idx, y);
		idx --;
	}
	return out;
}

int main() {
	cin >> h >> w >> n;

	foru(i, n) cin >> d[i];

	if(w%d[0] != 0 || h%d[0] != 0){
		cout << -1; return 0;
	}

	cout << solve(h, w);
	
	return 0;
}