#include<cstdio>
#include<iostream>
#include<cstring>
#include<algorithm>
#include<cmath>
#include<cstdlib>
#include<cmath>
#include<queue>
#include<map>
#include<iomanip>
using namespace std;
long long int n,d,a,b;
int main()
{
	freopen("mountain.in","r",stdin);
	freopen("mountain.out","w",stdout);
	cin>>n>>d>>a>>b;
	n--;
	if(a>b)
{
		long long int V;
		V=a;
		a=b;
		b=V;
}
	long long  G;
    long long ans;
	G=b-a;
	n-=(G/d);
	G%=d;
	if(n==0)
	{cout<<b<<endl;
		return 0;
	}
	else
	{
		if(G==0)
		{cout<<d*(n/2)+b<<endl;
			return 0;
		}
	}
	if(n%2==0) cout<<d*(n/2)+a<<endl;
	else cout<<d*(n/2)+b<<endl;
return 0;
}