#include <bits/stdc++.h>
using namespace std;
long long n,a,k,b,sum;
int main(){
//	1000000
	cin>>n>>a>>k>>b;
	if(n%2!=0){
		cout<<-1;
		return 0;
	}
    n/=2;
    sum=n*a;
    if(k<=(a*b)){
        if(n%k!=0){
            cout<<min(2*b*(n/k+1),2*(n/k*b+(n%k)*a));
        }
        else{
            cout<<2*n/k*b;
        }
    }
    else{
        cout<<sum*2;
    }
	return 0;
}