#include<bits/stdc++.h>
using namespace std;
int a,b,n,k;
int main(){
	cin>>n>>a>>k>>b;
	int x=((n%k==0?n/k:n/k+1)*b);
	int y=a*n;
	if(n%2==1)cout<<-1;
	else cout<<min(x,y);
}