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#include <bits/stdc++.h>

using namespace std;



int main(){
	
	long long w,h,n,a,b,x,p,q,wyn=0;
	
	cin>>h>>w;
	cin>>n;
	long long T[n];
	for(int i=0;i<n;i++){
		cin>>T[i];
	}
	a = h%T[0];
	b = w%T[0];
	if(a!=0 || b!=0){
		cout<<-1;
		return 0;
	}
	a = h/T[0];
	b = w/T[0];
	wyn = a * b;
	
	for(int i=1;i<n;i++){
		x = T[i]/T[i-1];
		x = x * x;
		a = h/T[i];
		b = w/T[i];
		p = a*b;
		q = p * x;
		wyn -= q;
		wyn += p;
	}
	
	cout<<wyn;
	
}