#include<stdio.h>
#include<fstream>
using namespace std;
int main()
{
	//freopen("mountain.in","r",stdin);
	//freopen("mountain.out","w",stdout);
	ifstream fin("mountain.in");
	ofstream fout("mountain.out");
	long long a;
	int b,c,d,e,f,z;
	fin>>a>>b>>c>>d;
	//scanf("%d%d%d%d",&a,&b,&c,&d);
	a--;
	if(c<=d)
	{
		e=d-c;
		if((e%b)!=0)
			f=(e/d)+1;
		if(e%b==0)
			f=e/d;
		if((a-f)%2==0)
			fout<<e+b*(a-f)+c;
			//printf("%d",e+b*(a-f)+c);
		if((a-f)%2==1)
			fout<<e+b*(a-f-1)+(b-e%d)+c;
			//printf("%d",e+b*(a-f-1)+(b-e%d)+c);
	}
	if(c>d)
	{
		z=c;
		c=d;
		d=z;
		e=d-c;
		if((e%b)!=0)
			f=(e/d)+1;
		if(e%b==0)
			f=e/d;
		if((a-f)%2==0)
			fout<<e+b*(a-f)+c;
			//printf("%d",e+b*(a-f)+c);
		if((a-f)%2==1)
			fout<<e+b*(a-f-1)+(b-e%d)+c;
			//printf("%d",e+b*(a-f-1)+(b-e%d)+c);
	}
return 0;
}