diff --git a/input/day05.txt b/input/day05.txt
new file mode 100644
index 0000000..410cf15
--- /dev/null
+++ b/input/day05.txt
@@ -0,0 +1 @@
+dabAcCaCBAcCcaDA
\ No newline at end of file
diff --git a/src/main.rs b/src/main.rs
index 0dbbe0c..58844d7 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,4 +1,4 @@
 fn main() {
-    // aoc_2018::tasks::day04::task1();
-    aoc_2018::tasks::day04::task2();
+    // aoc_2018::tasks::day05::task1();
+    aoc_2018::tasks::day05::task2();
 }
diff --git a/src/tasks/day05.rs b/src/tasks/day05.rs
new file mode 100644
index 0000000..bc4116e
--- /dev/null
+++ b/src/tasks/day05.rs
@@ -0,0 +1,51 @@
+use utils;
+
+pub fn task1() {
+    let mut input = utils::read_file("input/day05.txt");
+    input = reduce(input);
+
+    println!("resulting polymer: {}", input); //11591 too high
+    println!("RESULT: {}", input.len());
+}
+
+pub fn task2() {
+    let input = utils::read_file("input/day05.txt");
+    let best = "abcdefghijklmnopqrstuvwxyz"
+        .chars()
+        .map(|c| {
+            (
+                c,
+                reduce(
+                    input
+                        .chars()
+                        .filter(|ch| ch.to_lowercase().next().unwrap() != c)
+                        .collect(),
+                ),
+            )
+        }).max_by_key(|it| it.1.len())
+        .unwrap();
+
+    println!("Best: {} (length {})", best.0, best.1.len()); // 11184 too high
+}
+
+fn reduce(mut input: String) -> String {
+    let mut go_on = true;
+    while go_on {
+        go_on = false;
+        let mut index = 0;
+        while index < input.len() - 1 {
+            let a = input.chars().nth(index).unwrap();
+            let b = input.chars().nth(index + 1).unwrap();
+            if a.is_lowercase() && b.is_uppercase() && a == b.to_lowercase().next().unwrap()
+                || b.is_lowercase() && a.is_uppercase() && b == a.to_lowercase().next().unwrap()
+            {
+                // println!("{} and {} match", a, b);
+                input.remove(index);
+                input.remove(index);
+                go_on = true;
+            }
+            index += 1;
+        }
+    }
+    input
+}
diff --git a/src/tasks/mod.rs b/src/tasks/mod.rs
index e051fbd..f290640 100644
--- a/src/tasks/mod.rs
+++ b/src/tasks/mod.rs
@@ -2,3 +2,4 @@ pub mod day01;
 pub mod day02;
 pub mod day03;
 pub mod day04;
+pub mod day05;