import java.util.Scanner;

public class Yukicoder396 {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long N = sc.nextLong();
        long M = sc.nextLong();
        long X = sc.nextLong();
        long Y = sc.nextLong();

        long buf = X;
        while (buf > 0) {
            buf -= M * 2;
        }
        buf += M * 2;
        if (buf > M )   buf = (buf - M) * 2 - 1;
        
        boolean flip = false;
        long add1 = (M - buf) * 2 + 1;
        long add2 = M * 2 - add1;
        
        while (buf < N) {
            if (!flip) {
                buf += add1;
                flip = !flip;
            } else {
                buf += add2;    
                flip = !flip;
            }
            if (buf == Y) {
                System.out.println("YES");
                return;
            }
        }

        System.out.println("NO");
    }
}