代码 40分

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