$n, $m = gets.chomp.split(' ').map{|i| i.to_i(10)}
$x, $y = gets.chomp.split(' ').map{|i| i.to_i(10)}

def find_class (x, m, n)
x -= 1
d = (x / m) & 1
if d == 0 then
(x % m) + 1
else
n - (x % m)
end
end

if find_class($x, $m, $n) == find_class($y, $m, $n) then
puts "YES"
else
puts "NO"
end