#include<bits/stdc++.h>
using namespace std;
int n,a,k,b,ans=0,ans1=0,ans2=0,p[10];
int main(){
	cin>>n>>a>>k>>b;
	if(n%2!=0){
		cout<<-1;
		return 0;
	}
	int num=n/2;
	if(num>=k){
		 ans=(num-k-1)*a+b;
		 int f=num/(k-1);
		 ans1=f*b;
		 ans2=num*a;
		 p[0]=ans;
		 p[1]=ans1;
		 p[2]=ans2;
		 sort(p,p+3);
		 cout<<p[0];
		 return 0;
	}else{
		cout<<b;
		return 0;
	}
	return 0;
}