#include <iostream>
#include <cstdio>
using namespace std;

long long n,d,a,b,h;
long long m;
int main()
{
	freopen("mountain.in","r",stdin);
	freopen("mountain.out","w",stdout);
	
	cin>>n>>d>>a>>b;
	
	if (a>b)
	{
		int t=a; a=b; b=t;
	}
	m=(b-a)/d;
	n--;//减去第一分钟 
	n-=m;
	if (n%2!=0|| n%2==0 && (b-a)%d==0)
	{
		h=b+n/2*d;
	}
	else
	{
		h=b+(n-1)/2*d+(d-(b-a));
	}
	cout<<h<<endl;
	return 0;
}