diff --git a/day_09/first.rb b/day_09/first.rb
new file mode 100644
index 0000000..62df651
--- /dev/null
+++ b/day_09/first.rb
@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+input = File.readlines('./input').map(&:to_i)
+preamble = 25
+
+input.each_with_index do |_line, i|
+  next if i < preamble
+
+  unless input[i - preamble, preamble].combination(2).find { |a, b| a + b == input[i] }
+    puts input[i]
+    return
+  end
+end
diff --git a/day_09/input b/day_09/input
new file mode 100644
index 0000000..eb10606
--- /dev/null
+++ b/day_09/input
@@ -0,0 +1,1000 @@
+11
+1
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diff --git a/day_09/second.rb b/day_09/second.rb
new file mode 100644
index 0000000..17e21d3
--- /dev/null
+++ b/day_09/second.rb
@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+input = File.readlines('./input').map(&:to_i)
+preamble = 25
+invalid_num = 21_806_024
+
+input.each_with_index do |_line, i|
+  next if i < preamble
+
+  res = []
+
+  range = input[i - preamble, preamble]
+
+  range.each_index do |j|
+    res << range[i..-1] if range[i..-1]&.size.to_i > 1
+    preamble.times do |k|
+      res << range[j, k] if range[j, k]&.size > 1
+    end
+  end
+  out = res.uniq.select { |elems| elems.inject(:+) == invalid_num }
+  next if out.empty?
+
+  puts out.flatten.min + out.flatten.max
+  break
+end