#include<iostream>
using namespace std;
long T,X,A,Y,B;
int main()
{
	cin>>T>>X>>A>>Y>>B;
	long ans=9e18;
	for(int i=0;i<=B;i++)
	{
		long d=A*i;
		long now=X*i;
		if(d>T)
		{
			long diff=d-T;
			diff=(diff+B-1)/B;
			now+=Y*diff;
			d-=B*diff;
		}
		ans=min(ans,now+(T-d));
	}
	cout<<ans<<endl;
}