#include <cstdio>
#include <algorithm>

using namespace std;

long long n,d,a,b;

long long co(long long s,long long t){
	if(s>t)return co(t,s);
	if((t-s)%d==0)return (t-s)/d;
	else return (t-s)/d+1;
}

bool is_okay(long long h){
	if(co(a,h)+co(h,b)<n)return true;
	else return false;
}

int main(){
	long long l,r,mid;

	freopen("mountain.in","r",stdin);
	freopen("mountain.out","w",stdout);

	scanf("%lld%lld%lld%lld",&n,&d,&a,&b);
	l=max(a,b);
	r=l+n*d;
	while(r!=l){
		mid=(l+r)/2;
		if(is_okay(mid))l=mid+1;
		else r=mid;
	}
	printf("%lld\n",l-1);
	return 0;
}