#include<bits/stdc++.h>
using namespace std;
#define int long long
int pan(int i,int j)
{
	if (i%j!=0)
	{
		return i/j+1;
	}
	else
	{
		return i/j;
	}
}
signed main()
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	int n,a,k,b;
	cin>>n>>a>>k>>b;
	if (n%2)
	{
		cout<<-1;
	}
	else
	{
		if (a*n<=b*(pan(n/2,k)*2))
		{
			cout<<n*a;
		}
		else
		{
			cout<<b*(pan(n/2,k)*2);
		}
	}
	return 0; 
} 

~!@#$%^&*()_+{}|:"<>?`[]-=./;'