class D
  @i = 0 
  @c = 0 
  def initialize
  end 
  attr_accessor :i
  attr_accessor :c
end

m, n = gets.strip.split(" ").map { |x| x.to_i }
xx, yy = gets.strip.split(" ").map { |x| x.to_i }

# 上がったり下がったりするロジック
data = []
(0..m-1).map {|x| x.to_i }.each_with_index {|i, x|
  c = i % n 
  d = D.new
  d.i = x+1 
  if (i/n).to_i%2 == 0 then
    d.c = c 
  else
    d.c = n - c - 1 
  end 
  data.push(d)
}

xsan = data.select {|x| x.i == xx}.first
ysan = data.select {|x| x.i == yy}.first
if xsan.c == ysan.c then 
  puts "YES"
else 
  puts "NO"
end