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

  m *= 2
  if (xx - yy)%m == 0 or (xx + yy -2)%m = m - 1 then
    puts "YES"
  else
    puts "NO"
  end
end 

run()