#include<bits/stdc++.h>

using namespace std;
typedef long long ll;

const ll K = 30 + 5;
ll k = 0, d[K] = {};
ll ans = 0;

inline ll calc(ll n, ll m){
	for(ll i = k ; i >= 1 ; i --) if(d[i] <= n && d[i] <= m){
		ll x = n / d[i], y = m / d[i];
		return x * y + calc(x * d[i], m - y * d[i]) + calc(n - x * d[i], y * d[i]) + calc(n - x * d[i], m - y * d[i]);
	}
	return 0;
}

int main(){
	ll n = 0, m = 0;
	scanf("%lld %lld %lld", &n, &m, &k);
	for(ll i = 1 ; i <= k ; i ++) scanf("%lld", &d[i]);
	if(n % d[1] || m % d[1]){
		printf("-1");
		return 0;
	}
	printf("%lld", calc(n, m));
	return 0;
}