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