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#include <bits/stdc++.h>
#define FOR(i,p,k) for(int i=(p);i<=(k);++i)
#define REP(i,n) FOR(i,0,(n)-1)
#define ssize(x) (int((x).size()))
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define gc getchar_unlocked
using namespace std;
typedef long long ll;

template<typename T> void wczytaj(T &a){
	int c = gc();
	while(c < '0' || c > '9') c = gc();
	for(a = 0; c >= '0' && c <= '9'; c = gc()) a = a*10+c-'0';
}

void solve(){
	ll a, b; int n;
	wczytaj(a), wczytaj(b), wczytaj(n);
	ll wyn = 0ll, pop = 0ll;
	while(n--){
		int teraz;
		wczytaj(teraz);
		if(!pop){
			if(a%teraz || b%teraz) return void(printf("-1"));
			a /= teraz, b /= teraz;
		}
		else{
			ll t = teraz/pop;
			wyn += (a%t)*b, a -= a%t;
			wyn += (b%t)*a, b -= b%t;
			a /= t, b /= t;
		}
		pop = teraz;
	}
	wyn += a*b;
	printf("%lld", wyn);
}

int main(){
	solve();
	return 0;
}