#include<bits/stdc++.h>
using namespace std;
long long n,a,k,b,sum,ans,ans2;
int main()
{
	cin>>n>>a>>k>>b;
	if(n%2!=0)
	{
	    cout<<-1;
		return 0;
	}
	sum=n*a;
	n/=2;
	if(n%k==0)
	{
		ans2=n/k*b;			
	}
	else
	{
		ans=n/k*b+b;
		ans2=n/k*b+n%k*a;
		if(ans<=ans2)
		{
		    ans2=ans;
		}
	}
	if(ans2<sum)
	{
	    cout<<ans2*2;
	}
	if(ans2>sum)
	{
	    cout<<sum*2;
	}
	return 0;
}

就6号测试点WA了！！！