import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		long N = scan.nextInt();
		long M = scan.nextInt();
		long X = scan.nextInt();
		long Y = scan.nextInt();
		scan.close();
		if(M >= N) {
			System.out.println("NO");
			System.exit(0);
		}
		long x, y;
		if(X % M == 0) {
			if(X / M % 2 == 1) {
				x = M;
			}else {
				x = 1;
			}
		}else {
			if(X / M % 2 == 0) {
				x = X % M;
			}else {
				x = M - X % M + 1;
			}
		}
		if(Y % M == 0) {
			if(Y / M % 2 == 1) {
				y = M;
			}else {
				y = 1;
			}
		}else {
			if(Y / M % 2 == 0) {
				y = Y % M;
			}else {
				y = M - Y % M + 1;
			}
		}
		if(x == y) {
			System.out.println("YES");
		}else {
			System.out.println("NO");
		}
	}
}