#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <climits>
using namespace std;

long long d,a,b; 
long long n;
long long t1,t2;
long long t3,t4;
int main()
{
	freopen("mountain.in","r",stdin);
	freopen("mountain.out","w",stdout);
	//freopen("test.in","r",stdin);
	//freopen("test.out","w",stdout); 
	
	int s1; 
	cin>>n>>d>>a>>b;
	
	t1=(n-1)/2;
	//xiang shang xun dong de shi jian
	// wei zhi hui yuan lai
	s1=b-a;
	if((n-1)%2==0)
	{
		if(fabs(s1)>=d)
		{
			t2=s1/d;
		}
	}
	else
	{
		if(fabs(s1)>=d) t2=s1/d+1;
	}
	t4=(n-1+t2)/2;
	cout<<d*t4+a<<endl;
	return 0;
}