#include<bits/stdc++.h>
using namespace std;
int main(){
	long long n,a,k,b;
	cin>>n>>a>>k>>b;
	if(n%2==1){
		cout<<"-1";
		return 0;
	}
	if(b*1.0/k<a)cout<<n/2/k*b+n%k*a*2;
	else cout<<n/2*a*2;
	return 0;
}