diff --git a/input/2021/day1-sample1.txt b/input/2021/day1-sample1.txt
new file mode 100644
index 0000000..167e291
--- /dev/null
+++ b/input/2021/day1-sample1.txt
@@ -0,0 +1,10 @@
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diff --git a/input/2021/day1.txt b/input/2021/day1.txt
new file mode 100644
index 0000000..74af98e
--- /dev/null
+++ b/input/2021/day1.txt
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diff --git a/src/main/scala/de.qwertyuiop.aoc/2021/day1.scala b/src/main/scala/de.qwertyuiop.aoc/2021/day1.scala
index cd58684..ce85c3c 100644
--- a/src/main/scala/de.qwertyuiop.aoc/2021/day1.scala
+++ b/src/main/scala/de.qwertyuiop.aoc/2021/day1.scala
@@ -3,4 +3,15 @@ package de.qwertyuiop.aoc.`2021`
 import de.qwertyuiop.aoc.lib.*
 import cats.*, cats.implicits.given
 
-def day1(using InputSource): Unit = ???
+def day1(using InputSource): Unit =
+  val numbers = input(_.toInt)
+  println(countIncreases(numbers))
+  println(countIncreasesSmoothed(numbers, 3))
+
+/* part 1 */
+def countIncreases(input: List[Int]): Int = input.sliding(2).count(l => l(0) < l(1))
+
+/* part 2 */
+def countIncreasesSmoothed(input: List[Int], smoothingWindow: Int) =
+  countIncreases(input.sliding(smoothingWindow).map(_.sum).toList)
+