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