#include<bits/stdc++.h>
using namespace std;
int main(){
	long long n,a,b,k,sum=1e1000000,f;
	cin>>n>>a>>k>>b;
	if(n%2==1){
		printf("-1\n");
		return 0;
	}
	f=n/2;
	for(long i=0;i<n;i++){
		if((f-i)%k!=0){
			continue;
		}
		long long s=i*a+f/k*b;
		sum=min(sum,s);
	}
	printf("%lld\n",sum);
	return 0;
}

求调