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#pragma GCC optimize("O3")
#include <bits/stdc++.h>
#define fi first
#define se second
#define pn printf("\n")
#define ssize(x) int(x.size())
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
#define bitcount(x) __builtin_popcount(x)
#define clz(x) __builtin_clz(x)
#define ctz(x) __builtin_ctz(x)
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<int, ll> pil;
typedef pair<ll, int> pli;
typedef pair<ll, ll> pll;
typedef double db;
typedef long double ldb;
#define vv vector
/*void read(int &a){
		char c = getchar_unlocked(); a = 0;
		while(c<'0' || '9'<c) c = getchar_unlocked();
		while('0'<=c&&c<='9') a = (a<<3)+(a<<1)+c-'0', c = getchar_unlocked();
}*/
int inf = 2e09; ll infll = 2e18; int mod = (1<<23)*119+1;
int add(int a, int b){return a+b >= mod ? a+b - mod : a+b;}
int sub(int a, int b){return a-b < 0 ? a-b + mod : a-b;}
int mul(int a, int b){return int(a * ll(b) % mod);}
int fpow(int a, int b){
		int ret = 1;
		while(b){
				if(b & 1) ret = mul(ret, a);
				b >>= 1, a = mul(a, a);
		} return ret;
}
int inv(int a){ return fpow(a, mod-2); }
int coeff(int n, int k, vector<int> &fac, vector<int> &invfac){
		if(n < k) return 0;
		return mul(fac[n], mul(invfac[n-k], invfac[k]));
}
void calcfac(int n, vector<int> &fac, vector<int> &invfac){
		fac[0] = 1, invfac[0] = 1;
		for(int i = 1; i <= n; ++i) fac[i] = mul(fac[i-1], i);
		invfac[n] = inv(fac[n]);
		for(int i = n-1; i; --i) invfac[i] = mul(invfac[i+1], i+1);
}
void answer(){
		ll x, y; int n; scanf("%lld%lld%d", &x, &y, &n);
		vv<ll> t(n);
		for(int i = 0; i < n; ++i) scanf("%lld", &t[i]);
		if(x % t[0] || y % t[0]) return void(printf("-1\n"));
		ll result = (x/t[0])*ll(y/t[0]);
		x /= t[0], y /= t[0];
		for(int i = 1; i < n; ++i){
				ll k = t[i]/t[i-1];
				if(x < k || y < k) break;
				ll X = x-x%k, Y = y-y%k;
				result = result-X*Y+(x/k)*ll(y/k);
				x = x/k, y = y/k;
		}
		printf("%lld\n", result);
}
signed main(){
		int T = 1;
		//~ scanf("%d", &T);
		//~ ios_base::sync_with_stdio(0); cin.tie(0); //cin >> T;
		for(++T; --T; ) answer();
		return 0;
}