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