diff --git a/data/input-15.txt b/data/input-15.txt
new file mode 100644
index 0000000..75e31b4
--- /dev/null
+++ b/data/input-15.txt
@@ -0,0 +1 @@
+15,5,1,4,7,0
diff --git a/data/output-15.txt b/data/output-15.txt
new file mode 100644
index 0000000..10fcccf
--- /dev/null
+++ b/data/output-15.txt
@@ -0,0 +1,2 @@
+1259
+689
diff --git a/src/day14.rs b/src/day14.rs
index 4718805..8563fe0 100644
--- a/src/day14.rs
+++ b/src/day14.rs
@@ -1,4 +1,4 @@
-// ==== day 12
+// ==== day 14
 
 use crate::utils;
 use anyhow::Result;
diff --git a/src/day15.rs b/src/day15.rs
new file mode 100644
index 0000000..548c330
--- /dev/null
+++ b/src/day15.rs
@@ -0,0 +1,41 @@
+// ==== day 15
+
+use crate::utils;
+use anyhow::Result;
+use std::collections::BTreeMap;
+
+fn play_speaking_game(input: &[i64], n: usize) -> i64 {
+    let mut seen: BTreeMap<i64, usize> = BTreeMap::new();
+    let mut last_insert: Option<usize> = None;
+
+    input.iter().enumerate().for_each(|(i, n)| {
+        last_insert = seen.insert(*n, i);
+    });
+
+    (seen.len()..n).fold(*input.last().unwrap(), |_, step| {
+        let num: i64 = match last_insert {
+            Some(prev_step) => (step - 1 - prev_step) as i64,
+            None => 0_i64,
+        };
+
+        last_insert = seen.insert(num, step);
+        num
+    })
+}
+
+// main
+pub fn run() -> Result<(i64, i64)> {
+    let data: Vec<i64> = utils::read_file("data/input-15.txt")?
+        .trim()
+        .split(',')
+        .map(|s| s.parse::<i64>().expect("parse error"))
+        .collect();
+    let answers = utils::read_i64s("data/output-15.txt")?;
+
+    let p1 = play_speaking_game(&data, 2020);
+    let p2 = play_speaking_game(&data, 30000000);
+
+    assert_eq!(p1, answers[0]);
+    assert_eq!(p2, answers[1]);
+    Ok((p1, p2))
+}
diff --git a/src/main.rs b/src/main.rs
index f818802..9c26cf3 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -13,6 +13,7 @@ mod day10;
 mod day11;
 mod day12;
 mod day14;
+mod day15;
 mod utils;
 
 // TODO
@@ -39,5 +40,6 @@ fn main() -> Result<()> {
     println!("Day 11: {:?}", day11::run()?);
     println!("Day 12: {:?}", day12::run()?);
     println!("Day 14: {:?}", day14::run()?);
+    println!("Day 15: {:?}", day15::run()?);
     Ok(())
 }