#include<bits/stdc++.h>
using namespace std;
int main()
{
	int n,a,k,b,t,sum=0;//a搬体力，k块，需b体力 
	    cin>>n>>a>>k>>b;
	if(n%2==0)
	 {
	  t=n/2;
	   if((float)a>(float)b/(float)k)
	        {sum=sum+(t/k)*b;t=t%k;}
	      else {cout<<n*a;return 0;}
	  if(t*a<b) {sum=sum+t*a;}
	    else    {sum=sum+b;}
	 }
	else {cout<<"-1";return 0;}
	 cout<<2*sum;
	return 0;
}