#include <cstdio>
#include <iostream>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <climits>

using namespace std;

long long fen,chushi,xiandu,jieshu;
int gaocha,shicha,p;
int ans;

int main()
{
	freopen("mountain.in","r",stdin);
	freopen("mountain.out","w",stdout);
	scanf("%d%d%d%d",&fen,&xiandu,&chushi,&jieshu);
	fen--;
	if(chushi>jieshu)
	{
		int t;
		t=chushi;chushi=jieshu;jieshu=t;
	}
	gaocha=jieshu-chushi;
	fen-=(gaocha/xiandu);
	gaocha%=xiandu;
	if(fen==0)
	{
		cout<<jieshu<<endl;
		return 0;
	}
	else
	{
		if(gaocha==0)
		{
			cout<<xiandu*(fen/2)+jieshu<<endl;
			return 0;
		}
	}
	if(fen%2==0)
	{
		cout<<chushi+xiandu*(fen/2)<<endl;
	}
	else
	{
		cout<<jieshu+xiandu*(fen/2)<<endl;
	}
	return 0;
}