diff --git a/2019/day4/part2.rb b/2019/day4/part2.rb
new file mode 100644
index 0000000..d303598
--- /dev/null
+++ b/2019/day4/part2.rb
@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+input = '' # TODO
+range = Range.new(*input.split('-').map(&:to_i))
+
+def pairwise(arr)
+  arr[0...-1].zip(arr[1..])
+end
+
+def runs(digits)
+  runs = []
+  ref = digits.first
+  run = ref.to_s
+  digits[1..].each do |d|
+    if d == ref
+      run += d.to_s
+    else
+      runs << run
+      ref = d
+      run = ref.to_s
+    end
+  end
+  runs << run
+end
+
+count = range.count do |n|
+  # 1. All numbers in this range are six digits
+  # 2. All numbers in this range are in this range
+
+  digits = n.digits.reverse
+
+  # 3. Two adjacent digits are the same
+  double = runs(digits).any? { |run| run.length == 2 }
+
+  # 4. Going from left to right, the digits *never decrease*
+  monotonic = pairwise(digits).none? { |pair| pair.last < pair.first }
+
+  double && monotonic
+end
+pp count