具体内容

#include <bits/stdc++.h>
using namespace std;
int main(){
    long long n,a,k,b,t1,t2,zn;
    cin>>n>>a>>k>>b;
    if(n%2!=0){
	   cout<<-1;
	   return 0;
	}
	if(a<=b*1.0/k){
		cout<<n*a;
		//return 0;
	}else{
		zn=n/2;
		t1=zn/k*b+n%k*a;
		t2=ceil(zn*1.0/k)*b;
		cout<<max(t1,t2)*2;
	}
	return 0;
}