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#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,n) for (int i=a;i<n;i++)
#define per(i,a,n) for (int i=n-1;i>=a;i--)
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define SZ(x) ((int)(x).size())
typedef vector<int> VI;
typedef basic_string<int> BI;
typedef long long ll;
typedef pair<int,int> PII;
typedef double db;
mt19937 mrand(1); 
const ll mod=1000000007;
int rnd(int x) { return mrand() % x;}
ll powmod(ll a,ll b) {ll res=1;a%=mod; assert(b>=0); for(;b;b>>=1){if(b&1)res=res*a%mod;a=a*a%mod;}return res;}
ll gcd(ll a,ll b) { return b?gcd(b,a%b):a;}
// head

int n;
ll h,w,a[40];
const ll inf=1ll<<60;
map<array<int,3>,ll> hs;
int main() {
	scanf("%lld%lld",&h,&w);
	scanf("%d",&n);
	rep(i,0,n) scanf("%lld",&a[i]);
	if (h%a[0]!=0||w%a[0]!=0) {
		puts("-1");
		return 0;
	}
	ll ans=0;
	rep(i,0,n-1) ans+=(h/a[i])*(w/a[i])-(h/a[i+1])*(w/a[i+1])*(a[i+1]/a[i])*(a[i+1]/a[i]);
	ans+=(h/a[n-1])*(w/a[n-1]);
	printf("%lld\n",ans);
}