#include<bits/stdc++.h>
using namespace std;
int main(){
	long long n,a,k,b,sum,ans,ans2;
	cin>>n>>a>>k>>b;
	if(n%2!=0)cout<<-1;return 0;
	sum=n*a;
	n/=2;
	if(n%k==0){
		ans2=n/k*b;			
	}else{
		ans=n/k*b+b;
		ans2=n/k*b+n%k*a;
		if(ans<=ans2)ans2=ans;
	}
	ans2*=2;
	if(ans2<sum)cout<<ans2;
	else if(ans2>sum)cout<<sum;
	return 0;
}