package no396;

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int m = sc.nextInt();
		int x = sc.nextInt();
		int y = sc.nextInt();
		int p1 = (x - 1) % (m * 2);
		int p2 = (y - 1) % (m * 2);
		System.out.println((p1 == p2 || p1 + p2 == m * 2 - 1) ? "YES" : "NO");
	}

}