求助

#include<bits/stdc++.h>
using namespace std;
#define ll long long
int main(){
	std::ios::sync_with_stdio(0);
	ll n,a,k,b;
	cin>>n>>a>>k>>b;
	if(n%2==1) {
	cout<<-1;return 0;}
	if((n/2)%k==0){
		if((n/2)/k*b>=(n/2)*a){
			cout<<n*a;
		}else{
			cout<<(n/2)/k*b*2;
		}
	}else{
		ll q=n/2/k+1;
		ll p=q-1;
		cout<<min({q*b*2,n*a,(p*b+(n-p*k)*a)});
	}
}