import java.util.Scanner;

public class Yukicoder396 {

    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[] g = new int[N];
        int m = 0;
        boolean up = true;
        for (int i = 0; i < N; i++) {
            g[i] = m;
            if (up) {
                m++;
                if (m >= M) {
                    m--;
                    up = false;
                }
            } else {
                m--;
                if (m < 0) {
                    m++;
                    up = true;
                }
            }
        }
        System.out.println(g[X - 1] == g[Y - 1] ? "YES" : "NO");
    }
}