diff --git a/input/2021/day12-sample1.txt b/input/2021/day12-sample1.txt
new file mode 100644
index 0000000..6fd8c41
--- /dev/null
+++ b/input/2021/day12-sample1.txt
@@ -0,0 +1,7 @@
+start-A
+start-b
+A-c
+A-b
+b-d
+A-end
+b-end
diff --git a/input/2021/day12-sample2.txt b/input/2021/day12-sample2.txt
new file mode 100644
index 0000000..62cc714
--- /dev/null
+++ b/input/2021/day12-sample2.txt
@@ -0,0 +1,10 @@
+dc-end
+HN-start
+start-kj
+dc-start
+dc-HN
+LN-dc
+HN-end
+kj-sa
+kj-HN
+kj-dc
diff --git a/input/2021/day12-sample3.txt b/input/2021/day12-sample3.txt
new file mode 100644
index 0000000..65f3833
--- /dev/null
+++ b/input/2021/day12-sample3.txt
@@ -0,0 +1,18 @@
+fs-end
+he-DX
+fs-he
+start-DX
+pj-DX
+end-zg
+zg-sl
+zg-pj
+pj-he
+RW-he
+fs-DX
+pj-RW
+zg-RW
+start-pj
+he-WI
+zg-he
+pj-fs
+start-RW
diff --git a/input/2021/day12.txt b/input/2021/day12.txt
new file mode 100644
index 0000000..ad9b759
--- /dev/null
+++ b/input/2021/day12.txt
@@ -0,0 +1,22 @@
+xx-xh
+vx-qc
+cu-wf
+ny-LO
+cu-DR
+start-xx
+LO-vx
+cu-LO
+xx-cu
+cu-ny
+xh-start
+qc-DR
+vx-AP
+end-LO
+ny-DR
+vx-end
+DR-xx
+start-DR
+end-ny
+ny-xx
+xh-DR
+cu-xh
diff --git a/src/main/scala/de.qwertyuiop.aoc/2021/day12.scala b/src/main/scala/de.qwertyuiop.aoc/2021/day12.scala
index 3c98a57..500f675 100644
--- a/src/main/scala/de.qwertyuiop.aoc/2021/day12.scala
+++ b/src/main/scala/de.qwertyuiop.aoc/2021/day12.scala
@@ -3,4 +3,24 @@ package de.qwertyuiop.aoc.`2021`
 import de.qwertyuiop.aoc.lib.*
 import cats.*, cats.implicits.given
 
-def day12(using InputSource): Unit = ???
+def day12(using InputSource): Unit =
+  val edges = input(_.splitOnce("-").getOrElse(sys.error("Invalid input")))
+    .flatMap((s,t) => List((s,t), (t,s)))
+    .groupMapReduce(_(0))(e => Set(e(1)))(_ | _)
+
+  def countPaths(edges: Map[String, Set[String]], allowSmallDoubleVisit: Boolean): Int=
+    def rec(pathHead: String, visitedSmall: Set[String], canStillDoubleVisit: Boolean): Int=
+      val neighbours = edges(pathHead).filter(n => n != "start" && (canStillDoubleVisit || !visitedSmall.contains(n)))
+      neighbours.toVector.map(n =>
+        if n == "end" then 1
+        else if n.head.isLower then
+          rec(n, visitedSmall + n, !visitedSmall.contains(n) && canStillDoubleVisit)
+        else
+          rec(n, visitedSmall, canStillDoubleVisit)
+      ).sum
+    rec("start", Set("start"), allowSmallDoubleVisit)
+
+  val allPaths = countPaths(edges, false)
+  val allPathsWithDoubleVisit = countPaths(edges, true)
+  println(s"Found ${allPaths} paths with single visits on small caves only")
+  println(s"Found ${allPathsWithDoubleVisit} paths with one double visit")