#!/usr/bin/env python
#coding:UTF-8

N,M = map(int,raw_input().split())
X,Y = map(int,raw_input().split())
sw = 0
i = 0

while i < N:
     if sw == 0:
          for j in range(1,M + 1):
               if i + 1 == X:
                    XC = j
               if i + 1 == Y:
                    YC = j
               i  = i + 1
          sw = 1
     elif sw == 1:
          for j in range(M,0):
               if i + 1 == X:
                    XC = j
               if i + 1 == Y:
                    YC = j
               i  = i + 1
          sw = 0

if XC == YC:
     print "YES"
else:
     print "NO"